Use Windows Vista API to get the user's home directory

Summary: This patch replaces usage of deprecated SHGetFolderPathW with SHGetKnownFolderPath. The usage of SHGetKnownFolderPath is wrapped to allow queries for other "known" folders in the near future.

Reviewers: aaron.ballman, gbedwell

Subscribers: chapuni, llvm-commits

Differential Revision: http://reviews.llvm.org/D13753

llvm-svn: 250501