Implement flock for Windows in compiler-rt

Summary:
This patch implements flock for Windows, needed to make gcda writing work in a multiprocessing scenario.

Fixes https://bugs.llvm.org/show_bug.cgi?id=34923.

Reviewers: zturner

Reviewed By: zturner

Subscribers: rnk, zturner, llvm-commits

Differential Revision: https://reviews.llvm.org/D38891

llvm-svn: 317705