llvm-config: Add preliminary Windows support

Summary:
This patch adds Windows support for a few of the llvm-config commands,
including cflags, ldflags, libs, and system-libs.

Currently llvm-config is untested, so this patch adds tests for the
commands that it fixes as well.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16762

llvm-svn: 260263