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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@309795 91177308-0d34-0410-b5e6-96231b3b80d8