[Support] Extend sys::path with user_cache_directory function.

Summary:
The new function sys::path::user_cache_directory tries to discover
a directory suitable for cache storage for current system user.

On Windows and Darwin it returns a path to system-specific user cache directory.

On Linux it follows XDG Base Directory Specification, what is:
- use non-empty $XDG_CACHE_HOME env var,
- use $HOME/.cache.

Reviewers: chapuni, aaron.ballman, rafael

Subscribers: rafael, aaron.ballman, llvm-commits

Differential Revision: http://reviews.llvm.org/D13801

llvm-svn: 251784