[OpenMP] Context selector extensions for template functions

With this extension the effects of `omp begin declare variant` will be
applied to template function declarations. The behavior is opt-in and
controlled by the `extension(allow_templates)` trait. While generally
useful, this will enable us to implement complex math function calls by
overloading the templates of the standard library with the ones in
libc++.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D85735