[OpenMP] Allow traits for the OpenMP context selector `isa`

It was unclear what `isa` was supposed to mean so we did not provide any
traits for this context selector. With this patch we will allow *any*
string or identifier. We use the target attribute and target info to
determine if the trait matches. In other words, we will check if the
provided value is a target feature that is available (at the call site).

Fixes PR46338

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D83281