Replace clang::FileData with llvm::vfs::Status

Summary:
FileData was only ever used as a container for the values in
llvm::vfs::Status, so they might as well be consolidated.

The `InPCH` member was also always set to false, and unused.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58924

llvm-svn: 355368