Apply simulate-remapped-rust-src-base even remap-debuginfo is set in config.toml

This is really a mess. Here is the situation before this change:

- UI tests depend on not having `rust-src` available. In particular, <3f374128ee/tests/ui/tuple/wrong_argument_ice.stderr (L7-L8)> is depending on the `note` being a single line and not showing the source code.
- When `download-rustc` is disabled, we pass `-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX` `-Ztranslate-remapped-path-to-local-path=no`, which changes the diagnostic to something like `  --> /rustc/FAKE_PREFIX/library/alloc/src/collections/vec_deque/mod.rs:1657:12`
- When `download-rustc` is enabled, we still pass those flags, but they no longer have an effect. Instead rustc emits diagnostic paths like this: `  --> /rustc/39c6804b92aa202369e402525cee329556bc1db0/library/alloc/src/collections/vec_deque/mod.rs:1657:12`. Notice how there's a real commit and not `FAKE_PREFIX`. This happens because we set `CFG_VIRTUAL_RUST_SOURCE_BASE_DIR` during bootstrapping for CI artifacts, and rustc previously didn't allow for `simulate-remapped` to affect paths that had already been remapped.
- Pietro noticed this and decided the right thing was to normalize `/rustc/<commit>` to `$SRC_DIR` in compiletest: 470423c3d2
- After my change to `x test core`, which rebuilds stage 2 std from source so `build/stage2-std` and `build/stage2` use the same `.rlib` metadata, the compiler suddenly notices it has sources for `std` available and prints those in the diagnostic, causing the test to fail.

This changes `simulate-remapped-rust-src-base` to support remapping paths that have already been remapped, unblocking download-rustc.

Unfortunately, although this fixes the specific problem for
download-rustc, it doesn't seem to affect all the compiler's
diagnostics. In particular, various `mir-opt` tests are failing to
respect `simulate-remapped-path-prefix` (I looked into fixing this but
it seems non-trivial). As a result, we can't remove the normalization in
compiletest that maps `/rustc/<commit>` to `$SRC_DIR`, so this change is
currently untested anywhere except locally.

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1458,9 +1458,8 @@ fn filter<'a>(sess: &Session, path: Option<&'a Path>) -> Option<&'a Path> {
                     ..
                 } = source_file_to_import;
 
-                // If this file is under $sysroot/lib/rustlib/src/ but has not been remapped
-                // during rust bootstrapping by `remap-debuginfo = true`, and the user
-                // wish to simulate that behaviour by -Z simulate-remapped-rust-src-base,
+                // If this file is under $sysroot/lib/rustlib/src/
+                // and the user wish to simulate remapping with -Z simulate-remapped-rust-src-base,
                 // then we change `name` to a similar state as if the rust was bootstrapped
                 // with `remap-debuginfo = true`.
                 // This is useful for testing so that tests about the effects of
@@ -1468,12 +1467,18 @@ fn filter<'a>(sess: &Session, path: Option<&'a Path>) -> Option<&'a Path> {
                 // compiler is bootstrapped.
                 if let Some(virtual_dir) = &sess.opts.unstable_opts.simulate_remapped_rust_src_base
                 && let Some(real_dir) = &sess.opts.real_rust_source_base_dir
-                && let rustc_span::FileName::Real(ref mut old_name) = name
-                && let rustc_span::RealFileName::LocalPath(local) = old_name {
+                && let rustc_span::FileName::Real(ref mut old_name) = name {
+                    let relative_path = match old_name {
+                        rustc_span::RealFileName::LocalPath(local) => local.strip_prefix(real_dir).ok(),
+                        rustc_span::RealFileName::Remapped { virtual_name, .. } => {
+                            option_env!("CFG_VIRTUAL_RUST_SOURCE_BASE_DIR").and_then(|virtual_dir| virtual_name.strip_prefix(virtual_dir).ok())
+                        }
+                    };
+                    debug!(?relative_path, ?virtual_dir, "simulate_remapped_rust_src_base");
                     for subdir in ["library", "compiler"] {
-                        if let Ok(rest) = local.strip_prefix(real_dir.join(subdir)) {
+                        if let Some(rest) = relative_path.and_then(|p| p.strip_prefix(subdir).ok()) {
                             *old_name = rustc_span::RealFileName::Remapped {
-                                local_path: None,
+                                local_path: None, // FIXME: maybe we should preserve this?
                                 virtual_name: virtual_dir.join(subdir).join(rest),
                             };
                             break;

tests/ui/track-diagnostics/track6.rs

@@ -1,6 +1,9 @@
 // compile-flags: -Z track-diagnostics
 // error-pattern: created at
 
+// Normalize the emitted location so this doesn't need
+// updating everytime someone adds or removes a line.
+// normalize-stderr-test ".rs:\d+:\d+" -> ".rs:LL:CC"
 
 
 pub trait Foo {

tests/ui/track-diagnostics/track6.stderr

@@ -1,9 +1,9 @@
 error[E0658]: specialization is unstable
-  --> $DIR/track6.rs:11:5
+  --> $DIR/track6.rs:LL:CC
    |
 LL |     default fn bar() {}
    |     ^^^^^^^^^^^^^^^^^^^
--Ztrack-diagnostics: created at $COMPILER_DIR/rustc_session/src/parse.rs:93:5
+-Ztrack-diagnostics: created at $COMPILER_DIR/rustc_session/src/parse.rs:LL:CC
    |
    = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
    = help: add `#![feature(specialization)]` to the crate attributes to enable