[MSVC Compat] Only warn for unknown clang-cl arguments

Summary:
MSVC's driver accepts all unknown arguments but warns about them.  clang
by default rejects all unknown arguments.  This causes issues
specifically with build systems such as autoconf which liberally pass
things such as $LDFLAGS to the compiler and expect everything to work.
This patch teaches clang-cl to ignore unknown driver arguments.

Reviewers: rnk

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16511

llvm-svn: 258720