[clang] Fill RealPathName for virtual files.

Summary:
Absolute path information for virtual files were missing even if we
have already stat'd the files. This patch puts that information for virtual
files that can succesffully be stat'd.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D55054

llvm-svn: 348006