[clang] Use the VFS to check the system framework marker (llvm#160946)

This PR uses the VFS/`FileManager` to check the system framework marker
instead of going straight to the real file system. This matches the
behavior of other input files of the compiler.

Author: jansvoboda11

clang/lib/Lex/HeaderSearch.cpp

@@ -672,9 +672,8 @@ OptionalFileEntryRef DirectoryLookup::DoFrameworkLookup(
     if (getDirCharacteristic() == SrcMgr::C_User) {
       SmallString<1024> SystemFrameworkMarker(FrameworkName);
       SystemFrameworkMarker += ".system_framework";
-      if (llvm::sys::fs::exists(SystemFrameworkMarker)) {
+      if (FileMgr.getOptionalFileRef(SystemFrameworkMarker))
         CacheEntry.IsUserSpecifiedSystemFramework = true;
-      }
     }
   }