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

[dsanders]

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

[EricWF]

Obviously LGTM. @hans I'm OK with this going into 3.8.

[dsanders]

Thanks. Given that you're the patch author should I wait for someone else to LGTM too?

[EricWF]

I'm OK signing off on this myself.

test/libcxx/test/format.py

class LibcxxTestFormat(object):
     def _evaluate_fail_test(self, test):
         source_path = test.getSourcePath()
         with open(source_path, 'r') as f:
             contents = f.read()
         verify_tags = ['expected-note', 'expected-remark', 'expected-warning',
                        'expected-error', 'expected-no-diagnostics']
         use_verify = self.use_verify_for_fail and \
                      any([tag in contents for tag in verify_tags])
-        extra_flags = []
+        extra_flags = ['-fsyntax-only']
         if use_verify:
             extra_flags += ['-Xclang', '-verify',
                             '-Xclang', '-verify-ignore-unexpected=note']
         cmd, out, err, rc = self.cxx.compile(source_path, out=os.devnull,
                                              flags=extra_flags)
         expected_rc = 0 if use_verify else 1
         if rc == expected_rc:
             return lit.Test.PASS, ''
         else:
             report = libcxx.util.makeReport(cmd, out, err, rc)
             report_msg = ('Expected compilation to fail!' if not use_verify else
                           'Expected compilation using verify to pass!')
             return lit.Test.FAIL, report + report_msg + '\n'