[Support] Add fs::getUmask() function and change fs::setPermissions

Summary: This patch changes fs::setPermissions to optionally set permissions while respecting the umask. It also adds the function fs::getUmask() which returns the current umask.

Reviewers: jhenderson, rupprecht, aprantl, lhames

Reviewed By: jhenderson, rupprecht

Subscribers: sanaanajjar231288, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63583

llvm-svn: 364621