Use VFS operations in FileManager::makeAbsolutePath.

Summary: It used to call into llvm::sys::fs::make_absolute.

Reviewers: akyrtzi, erikjv, bkramer, krasimir, klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D36155

llvm-svn: 309795