Add support to clang-cl driver for /GS switch

Summary:
This patch is adding command-line support for the MSVC buffer security check.

The buffer security check is turned on with the '/GS' compiler switch.
https://msdn.microsoft.com/en-us/library/8dbf701c.aspx

The MSVC buffer security check in implemented here:
http://reviews.llvm.org/D20346

Reviewers: hans, rnk

Subscribers: chrisha, cfe-commits, rnk, hans, thakis

Differential Revision: http://reviews.llvm.org/D20347

llvm-svn: 272832