[clang][DependencyScanner] Cache modulemap stat failures

[Bigcheese]

Summary

Add module.modulemap as a file we cache stat failures for as there
are a lot of stats for this file.

Clang currently uses the files it should minimize as a proxy for files
it should cache stat failures for, but really we should cache stat
failures for all paths we don't expect to change during the build.
Unfortunately the VFS API does not know _why_ clang is trying to stat
a path, so we use the filename as a proxy.

[Bigcheese]

Forgot that Filename is actually a path, so call llvm::sys::path::filename.

[Bigcheese]

Also handle module.map.

clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp

return llvm::StringSwitch<bool>(Ext)
       .CasesLower(".def", ".inc", true)
       .Default(false);
 }
 
 static bool shouldCacheStatFailures(StringRef Filename) {
   StringRef Ext = llvm::sys::path::extension(Filename);
   if (Ext.empty())
     return false; // This may be the module cache directory.
-  // Only cache stat failures on source files.
+  // Only cache stat failures on files that are not expected to change during
+  // the build.
+  if (llvm::sys::path::filename(Filename) == "module.modulemap")
+    return true;
   return shouldScanForDirectivesBasedOnExtension(Filename);
 }
 
 bool DependencyScanningWorkerFilesystem::shouldScanForDirectives(
     StringRef Filename) {
   return shouldScanForDirectivesBasedOnExtension(Filename);
 }