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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212541 91177308-0d34-0410-b5e6-96231b3b80d8