[Clang] [Diagnostics] Simplify filenames that contain '..' (#143520)

This can significantly shorten file paths to standard library headers,
e.g. on my system, `<ranges>` is currently printed as
```console
/usr/lib/gcc/x86_64-redhat-linux/15/../../../../include/c++/15/ranges
```
but with this change, we instead print
```console
/usr/include/c++/15/ranges
```

This is of course just a heuristic; there are paths that would get longer
as a result of this, so we use whichever path ends up being shorter.

@AaronBallman pointed out that this might be problematic for network
file systems since path resolution might take a while, so this is enabled 
only for paths that are part of a local filesystem—though not on Windows
since there we noticed that the check itself is slow.

The file names are cached in `SourceManager`.

2 files changed, 2 insertions, 63 deletions

diff --git a/clang/lib/Frontend/SARIFDiagnostic.cpp b/clang/lib/Frontend/SARIFDiagnostic.cpp
index ac27d74..f971401 100644
--- a/clang/lib/Frontend/SARIFDiagnostic.cpp
+++ b/clang/lib/Frontend/SARIFDiagnostic.cpp
@@ -163,36 +163,7 @@ SARIFDiagnostic::addDiagnosticLevelToRule(SarifRule Rule,
 
 llvm::StringRef SARIFDiagnostic::emitFilename(StringRef Filename,
                                               const SourceManager &SM) {
-  if (DiagOpts.AbsolutePath) {
-    auto File = SM.getFileManager().getOptionalFileRef(Filename);
-    if (File) {
-      // We want to print a simplified absolute path, i. e. without "dots".
-      //
-      // The hardest part here are the paths like "<part1>/<link>/../<part2>".
-      // On Unix-like systems, we cannot just collapse "<link>/..", because
-      // paths are resolved sequentially, and, thereby, the path
-      // "<part1>/<part2>" may point to a different location. That is why
-      // we use FileManager::getCanonicalName(), which expands all indirections
-      // with llvm::sys::fs::real_path() and caches the result.
-      //
-      // On the other hand, it would be better to preserve as much of the
-      // original path as possible, because that helps a user to recognize it.
-      // real_path() expands all links, which is sometimes too much. Luckily,
-      // on Windows we can just use llvm::sys::path::remove_dots(), because,
-      // on that system, both aforementioned paths point to the same place.
-#ifdef _WIN32
-      SmallString<256> TmpFilename = File->getName();
-      llvm::sys::fs::make_absolute(TmpFilename);
-      llvm::sys::path::native(TmpFilename);
-      llvm::sys::path::remove_dots(TmpFilename, /* remove_dot_dot */ true);
-      Filename = StringRef(TmpFilename.data(), TmpFilename.size());
-#else
-      Filename = SM.getFileManager().getCanonicalName(*File);
-#endif
-    }
-  }
-
-  return Filename;
+  return SM.getNameForDiagnostic(Filename, DiagOpts);
 }
 
 /// Print out the file/line/column information and include trace.
diff --git a/clang/lib/Frontend/TextDiagnostic.cpp b/clang/lib/Frontend/TextDiagnostic.cpp
index ccdd59d..e5bf867 100644
--- a/clang/lib/Frontend/TextDiagnostic.cpp
+++ b/clang/lib/Frontend/TextDiagnostic.cpp
@@ -738,39 +738,7 @@ void TextDiagnostic::printDiagnosticMessage(raw_ostream &OS,
 }
 
 void TextDiagnostic::emitFilename(StringRef Filename, const SourceManager &SM) {
-#ifdef _WIN32
-  SmallString<4096> TmpFilename;
-#endif
-  if (DiagOpts.AbsolutePath) {
-    auto File = SM.getFileManager().getOptionalFileRef(Filename);
-    if (File) {
-      // We want to print a simplified absolute path, i. e. without "dots".
-      //
-      // The hardest part here are the paths like "<part1>/<link>/../<part2>".
-      // On Unix-like systems, we cannot just collapse "<link>/..", because
-      // paths are resolved sequentially, and, thereby, the path
-      // "<part1>/<part2>" may point to a different location. That is why
-      // we use FileManager::getCanonicalName(), which expands all indirections
-      // with llvm::sys::fs::real_path() and caches the result.
-      //
-      // On the other hand, it would be better to preserve as much of the
-      // original path as possible, because that helps a user to recognize it.
-      // real_path() expands all links, which sometimes too much. Luckily,
-      // on Windows we can just use llvm::sys::path::remove_dots(), because,
-      // on that system, both aforementioned paths point to the same place.
-#ifdef _WIN32
-      TmpFilename = File->getName();
-      llvm::sys::fs::make_absolute(TmpFilename);
-      llvm::sys::path::native(TmpFilename);
-      llvm::sys::path::remove_dots(TmpFilename, /* remove_dot_dot */ true);
-      Filename = StringRef(TmpFilename.data(), TmpFilename.size());
-#else
-      Filename = SM.getFileManager().getCanonicalName(*File);
-#endif
-    }
-  }
-
-  OS << Filename;
+  OS << SM.getNameForDiagnostic(Filename, DiagOpts);
 }
 
 /// Print out the file/line/column information and include trace.