Do not resolve directory junctions for `-fdiagnostics-absolute-paths` on Windows.

If the source file path contains directory junctions, and we resolve them when
printing diagnostic messages, these paths look independent for an IDE.
For example, both Visual Studio and Visual Studio Code open separate editors
for such paths, which is not only inconvenient but might even result in losing
changes made in one of them.

Differential Revision: https://reviews.llvm.org/D59415

llvm-svn: 361598