Add regression test for generics reexport of private import

tests/rustdoc/issue-81141-private-reexport-in-public-api-generics.rs

@@ -0,0 +1,13 @@
+#![crate_name = "foo"]
+
+use crate::bar::Foo as Alias;
+
+pub mod bar {
+    pub struct Foo<'a, T>(&'a T);
+}
+
+// @has "foo/fn.foo.html"
+// @has - '//*[@class="rust item-decl"]/code' "pub fn foo<'a, T>(f: Foo<'a, T>) -> Foo<'a, usize>"
+pub fn foo<'a, T>(f: Alias<'a, T>) -> Alias<'a, usize> {
+    Alias(&0)
+}