Update some tests after r366702

Summary:
r366702 added a set of new clang-cl -- specific openmp flags together with tests.
The way the newly added tests work is problematic: consider for example this
asertion:
```
// RUN: %clang_cl --target=x86_64-windows-msvc /openmp -### -- %s 2>&1 | FileCheck --check-prefix=CHECK-CC1-OPENMP %s
...
// CHECK-CC1-OPENMP: "-fopenmp"
```

It asserts that an `/openmp` flag should expand into `-fopenmp`. This however
depends on the default value of Clang's CLANG_DEFAULT_OPENMP_RUNTIME value.
Indeed, the code that adds `-fopenmp` to the output only does it if the default
runtime is `libomp` or `libiomp5`, not when it is `libgomp`.

I've updated the tests to not depend on the default value of this setting by
specifying the runtime to use explicitly in each assertion.

Reviewers: jdoerfert

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65272

llvm-svn: 367012