[clang-move] Compare with real paths of symlinks

Summary: MakeAbsolutePath does wrong things with symlinks previously. When comparing with a symlink, we need to compare with the real path of it. This fixes issues when the build directory is a symlink.

Reviewers: ioeric

Subscribers: beanz, mgorny, cfe-commits, bkramer

Differential Revision: https://reviews.llvm.org/D25508

llvm-svn: 284020