[OpenMP] Context selector extensions for return value overloading

This extension allows to declare variants in between `omp begin/end
declare variant` that do not match the type of the existing function
with that name. Without this extension we would not find a base function
(with a compatible type), therefore create a new one, which would
cause conflicting declarations. With this extension we will not create
"missing" base functions, which basically renders these specializations
harmless. They will be generated but never called.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D85878