[clang-scan-deps] Fix contention when updating `TrackingStatistic`s in hot code paths in `FileManager`. (#88427)

`FileManager::getDirectoryRef()` and `FileManager::getFileRef()` are hot code paths in `clang-scan-deps`. These functions are updating on every call a few atomics related to printing statistics, which causes contention on high core count machines.

![Screenshot 2024-04-10
214123](https://github.com/llvm/llvm-project/assets/37383324/5756b1bc-cab5-4612-8769-ee7e03a66479)

![Screenshot 2024-04-10
214246](https://github.com/llvm/llvm-project/assets/37383324/3d560e89-61c7-4fb9-9330-f9e660e8fc8b)

![Screenshot 2024-04-10
214315](https://github.com/llvm/llvm-project/assets/37383324/006341fc-49d4-4720-a348-7af435c21b17)

After this patch we make the variables local to the `FileManager`.

In our test case, this saves about 49 sec over 1 min 47 sec of `clang-scan-deps` run time (1 min 47 sec before, 58 sec after). These figures are after applying my suggestion in https://github.com/llvm/llvm-project/pull/88152#issuecomment-2049803229, that is:
```
static bool shouldCacheStatFailures(StringRef Filename) {
  return true;
}
```
Without the above, there's just too much OS noise from the high volume of `status()` calls with regular non-modules C++ code. Tested on Windows with clang-cl.

1 files changed, 8 insertions, 6 deletions

diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
index cd520a6..143c043 100644
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -39,12 +39,6 @@ using namespace clang;
 
 #define DEBUG_TYPE "file-search"
 
-ALWAYS_ENABLED_STATISTIC(NumDirLookups, "Number of directory lookups.");
-ALWAYS_ENABLED_STATISTIC(NumFileLookups, "Number of file lookups.");
-ALWAYS_ENABLED_STATISTIC(NumDirCacheMisses,
-                         "Number of directory cache misses.");
-ALWAYS_ENABLED_STATISTIC(NumFileCacheMisses, "Number of file cache misses.");
-
 //===----------------------------------------------------------------------===//
 // Common logic.
 //===----------------------------------------------------------------------===//
@@ -656,6 +650,14 @@ StringRef FileManager::getCanonicalName(const void *Entry, StringRef Name) {
   return CanonicalName;
 }
 
+void FileManager::AddStats(const FileManager &Other) {
+  assert(&Other != this && "Collecting stats into the same FileManager");
+  NumDirLookups += Other.NumDirLookups;
+  NumFileLookups += Other.NumFileLookups;
+  NumDirCacheMisses += Other.NumDirCacheMisses;
+  NumFileCacheMisses += Other.NumFileCacheMisses;
+}
+
 void FileManager::PrintStats() const {
   llvm::errs() << "\n*** File Manager Stats:\n";
   llvm::errs() << UniqueRealFiles.size() << " real files found, "