Improve memory ownership of vfs::Files in the FileSystemStatCache by using std::unique_ptr

Spotted after a memory leak (due to the complexities of manual memory
management) was fixed in 212466.

llvm-svn: 212541
diff --git a/clang/lib/Frontend/CacheTokens.cpp b/clang/lib/Frontend/CacheTokens.cpp
index d30196d..14f7027 100644
--- a/clang/lib/Frontend/CacheTokens.cpp
+++ b/clang/lib/Frontend/CacheTokens.cpp
@@ -540,7 +540,8 @@
   ~StatListener() {}
 
   LookupResult getStat(const char *Path, FileData &Data, bool isFile,
-                       vfs::File **F, vfs::FileSystem &FS) override {
+                       std::unique_ptr<vfs::File> *F,
+                       vfs::FileSystem &FS) override {
     LookupResult Result = statChained(Path, Data, isFile, F, FS);
 
     if (Result == CacheMissing) // Failed 'stat'.