[Support] Tweak path::system_temp_directory() on Windows.

Summary:
This patch changes the behavior of path::system_temp_directory() on Windows to be closer to GetTempPath Windows API call. Enforces path separator to be the native one, makes path absolute, etc. GetTempPath is not used directly because of limitations/implementation bugs on Windows 7.

Windows specific unit tests are added. Most of them runs in separated process with modified environment variables.

This change fixes FileSystemTest.CreateDir unittest that had been failing when run from Unix-like shell on Windows (Unix-like path separator (/) used in env variables).

Reviewers: chapuni, rafael, aaron.ballman

Subscribers: rafael, llvm-commits

Differential Revision: http://reviews.llvm.org/D14231

llvm-svn: 253345