Introduce SourceManager::getDisplayNameForLoc

Add an abstraction to retrieve a name suitable for display for a given
source location for use with diagnostics, debug locations, and magic
file literals.

lib/Basic/SourceLoc.cpp

@@ -39,6 +39,27 @@ SourceLoc SourceManager::getCodeCompletionLoc() const {
       .getAdvancedLoc(CodeCompletionOffset);
 }
 
+StringRef SourceManager::getDisplayNameForLoc(SourceLoc Loc) const {
+  // Respect #line first
+  if (auto VFile = getVirtualFile(Loc))
+    return VFile->Name;
+
+  // Next, try the stat cache
+  auto Ident = getIdentifierForBuffer(findBufferContainingLoc(Loc));
+  auto found = StatusCache.find(Ident);
+  if (found != StatusCache.end()) {
+    return found->second.getName();
+  }
+
+  // Populate the cache with a (virtual) stat.
+  if (auto Status = FileSystem->status(Ident)) {
+    return (StatusCache[Ident] = Status.get()).getName();
+  }
+
+  // Finally, fall back to the buffer identifier.
+  return Ident;
+}
+
 unsigned
 SourceManager::addNewSourceBuffer(std::unique_ptr<llvm::MemoryBuffer> Buffer) {
   assert(Buffer);