[libcxx] Work around for clang calling GAS after having already failed.

Summary:
This is a workaround to a clang bug which causes libcxx tests to fail in the 3.8
release. The clang bug is currently being investigated. It seems that clang
does not stop after frontend errors when using -verify and -fno-integrated-as
(or when this is the default). This patch adds -fsyntax-only to prevent GAS
from being called, fixing the libcxx failures.

PR26277

Patch by Eric Fiselier

Reviewers: mclow.lists, hans, EricWF

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D16584

llvm-svn: 259046