[OPENMP] Fix mixture of omp and clang pragmas

Fixes PR45753

When a program that contains a loop to which both `omp parallel for`
pragma and `clang loop` pragma are associated is compiled with the
-fopenmp option, `clang loop` pragma did not take effect. The example
below should not be vectorized by the `clang loop` pragma but it was
actually vectorized. The cause is that `llvm.loop.vectorize.width`
was not output to the IR when -fopenmp is specified.

The fix attaches attributes if they exist for the loop.

[example.c]

```
int a[100], b[100];
void foo() {
  #pragma omp parallel for
  #pragma clang loop vectorize(disable)
  for (int i = 0; i < 100; i++)
    a[i] += b[i] * i;
}
```

[compile]

```
$ clang -O2 -fopenmp example.c -c -Rpass=vect
example.c:3:11: remark: vectorized loop (vectorization width: 4, interleaved count: 2) [-Rpass=loop-vectorize]
  #pragma omp parallel for
          ^
```

[IR with -fopenmp]

```
$ clang -O2 exmaple.c -S -emit-llvm -mllvm -disable-llvm-optzns -o - -fopenmp | grep 'vectorize\.width'
```

[IR with -fno-openmp]

```
$ clang -O2 example.c -S -emit-llvm -mllvm -disable-llvm-optzns -o - -fno-openmp | grep 'vectorize\.width'
!7 = !{!"llvm.loop.vectorize.width", i32 1}
```

Differential Revision: https://reviews.llvm.org/D79921