[Clang][OpenMP] Make test use clang_cc1 (#169233)

This test does not actually need to use the clang driver. Using the
driver means that the environment plays much more into the tests
results. We ran into a situation where the driver decided not to pass
-fopenmp to the cc1 invocation, causing the test to fail.

This also makes the test more consistent with the other OpenMP tests and
should make it slightly faster (no subprocess invocation).

clang/test/OpenMP/parallel_default_variableCategory_codegen.cpp

@@ -1,4 +1,4 @@
-// RUN: %clangxx -Xclang -verify -Wno-vla -fopenmp -fopenmp-version=60 -x c++ -S -emit-llvm %s -o - | FileCheck %s
+// RUN: %clang_cc1 -verify -fopenmp -fopenmp-version=60 -x c++ -emit-llvm %s -o - | FileCheck %s
 // expected-no-diagnostics
 #ifndef HEADER
 #define HEADER