[infra] Roll clang toolchain forward

This is to match the version used by Flutter.

R=asiva@google.com, rmacnak@google.com

Review-Url: https://codereview.chromium.org/3005903002 .

DEPS

@@ -36,7 +36,7 @@ vars = {
 
   # Revisions of GN related dependencies. This should match the revision
   # pulled by Flutter.
-  "buildtools_revision": "@057ef89874e3c622248cf99259434fdc683c4e30",
+  "buildtools_revision": "@5b8eb38aaf523f0124756454276cd0a5b720c17e",
 
   # Scripts that make 'git cl format' work.
   "clang_format_scripts_rev": "@c09c8deeac31f05bd801995c475e7c8070f9ecda",

build/config/compiler/BUILD.gn

@@ -329,6 +329,20 @@ config("compiler") {
     }
   }
 
+  # We want to force a recompile and relink of the world whenever our toolchain
+  # changes since artifacts from an older version of the toolchain may or may
+  # not be compatible with newer ones. To achieve this, we insert a synthetic
+  # define into the compile line.
+  if (is_clang && (is_linux || is_mac)) {
+    if (is_linux) {
+      toolchain_stamp_file = "//buildtools/linux64/toolchain.stamp"
+    } else {
+      toolchain_stamp_file = "//buildtools/mac/toolchain.stamp"
+    }
+    toolchain_version = read_file(toolchain_stamp_file, "trim string")
+    defines = [ "TOOLCHAIN_VERSION=$toolchain_version" ]
+  }
+
   # Assign any flags set for the C compiler to asmflags so that they are sent
   # to the assembler. The Windows assembler takes different types of flags
   # so only do so for posix platforms.

tools/buildtools/update.py

@@ -24,7 +24,7 @@ DEPOT_PATH = find_depot_tools.add_depot_tools_to_path()
 
 def Update():
   path = os.path.join(BUILDTOOLS, 'update.sh')
-  command = ['/bin/bash', path, '--toolchain', '--gn']
+  command = ['/bin/bash', path, '--clang', '--gn']
   return subprocess.call(command, cwd=DART_ROOT)