[OpenMP] Add variant extension that applies to declarations

This patch adds a new extension to the `omp begin / end declare variant`
support that causes it to apply to function declarations as well. This
is explicitly not done in the standard, but can be useful in some
situations so we should provide it as an extension. This will allow us
to uniquely bind and overload existing definitions with a simple
declaration using variants.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D124624