[clang][DependencyScanner] Remove unused -ivfsoverlay files (#73734)

`-ivfsoverlay` files are unused when building most modules. Enable
removing them by,
* adding a way to visit the filesystem tree with extensible RTTI to
  access each `RedirectingFileSystem`.
* Adding tracking to `RedirectingFileSystem` to record when it
  actually redirects a file access.
* Storing this information in each PCM.

Usage tracking is only enabled when iterating over the source manager
and affecting modulemaps. Here each path is stated to cause an access.
During scanning these stats all hit the cache.

1 files changed, 26 insertions, 0 deletions

diff --git a/llvm/lib/Support/VirtualFileSystem.cpp b/llvm/lib/Support/VirtualFileSystem.cpp
index d6c787c..c341170 100644
--- a/llvm/lib/Support/VirtualFileSystem.cpp
+++ b/llvm/lib/Support/VirtualFileSystem.cpp
@@ -480,6 +480,13 @@ OverlayFileSystem::getRealPath(const Twine &Path,
   return errc::no_such_file_or_directory;
 }
 
+void OverlayFileSystem::visitChildFileSystems(VisitCallbackTy Callback) {
+  for (IntrusiveRefCntPtr<FileSystem> FS : overlays_range()) {
+    Callback(*FS);
+    FS->visitChildFileSystems(Callback);
+  }
+}
+
 void OverlayFileSystem::printImpl(raw_ostream &OS, PrintType Type,
                                   unsigned IndentLevel) const {
   printIndent(OS, IndentLevel);
@@ -1581,6 +1588,13 @@ void RedirectingFileSystem::printEntry(raw_ostream &OS,
   }
 }
 
+void RedirectingFileSystem::visitChildFileSystems(VisitCallbackTy Callback) {
+  if (ExternalFS) {
+    Callback(*ExternalFS);
+    ExternalFS->visitChildFileSystems(Callback);
+  }
+}
+
 /// A helper class to hold the common YAML parsing state.
 class llvm::vfs::RedirectingFileSystemParser {
   yaml::Stream &Stream;
@@ -2263,12 +2277,18 @@ RedirectingFileSystem::makeCanonical(SmallVectorImpl<char> &Path) const {
 
 ErrorOr<RedirectingFileSystem::LookupResult>
 RedirectingFileSystem::lookupPath(StringRef Path) const {
+  // RedirectOnly means the VFS is always used.
+  if (UsageTrackingActive && Redirection == RedirectKind::RedirectOnly)
+    HasBeenUsed = true;
+
   sys::path::const_iterator Start = sys::path::begin(Path);
   sys::path::const_iterator End = sys::path::end(Path);
   llvm::SmallVector<Entry *, 32> Entries;
   for (const auto &Root : Roots) {
     ErrorOr<RedirectingFileSystem::LookupResult> Result =
         lookupPathImpl(Start, End, Root.get(), Entries);
+    if (UsageTrackingActive && Result && isa<RemapEntry>(Result->E))
+      HasBeenUsed = true;
     if (Result || Result.getError() != llvm::errc::no_such_file_or_directory) {
       Result->Parents = std::move(Entries);
       return Result;
@@ -2863,3 +2883,9 @@ recursive_directory_iterator::increment(std::error_code &EC) {
 
   return *this;
 }
+
+const char FileSystem::ID = 0;
+const char OverlayFileSystem::ID = 0;
+const char ProxyFileSystem::ID = 0;
+const char InMemoryFileSystem::ID = 0;
+const char RedirectingFileSystem::ID = 0;