[Support] Use GetTempDir to get the temporary dir path on Windows.
In general GetTempDir follows the same logic as the replaced code: checks env variables TMP, TEMP, USERPROFILE in order. However, it also perform other checks like making separators native (\), making the path absolute, etc.
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