[OpenMP] Try to find an existing base for `omp begin/end declare variant`

If we have a function definition in `omp begin/end declare variant` it
is a specialization of a base function with the same name and
"compatible" type. Before, we just created a declaration for the base.
With this patch we try to find an existing declaration first and only
create a new one if we did not find any with a compatible type. This is
preferable as we can tolerate slight mismatches, especially if the
specialized version is "more constrained", e.g., constexpr.

Reviewed By: mikerice

Differential Revision: https://reviews.llvm.org/D77252