[clang-cl] Make all sanitizer flags available in clang-cl

Summary:
Use a tablegen let {} block so that new sanitizer flags are available by
default in all driver modes. This should cut down on time wasted with
bugs like http://crbug.com/710928.

Reviewers: vitalybuka, hans

Subscribers: kcc, llvm-commits

Differential Revision: https://reviews.llvm.org/D31988

llvm-svn: 300122