Work around Clang driver segfault when --coverage is used with -c and /dev/null

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@284225 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/compiler.py

@@ -188,7 +188,7 @@ class CXXCompiler(object):
         # Add -Werror to ensure that an unrecognized flag causes a non-zero
         # exit code. -Werror is supported on all known compiler types.
         if self.type is not None:
-            flags += ['-Werror']
+            flags += ['-Werror', '-fsyntax-only']
         cmd, out, err, rc = self.compile(os.devnull, out=os.devnull,
                                          flags=flags)
         return rc == 0