[clang] WIP: Allow dynamic `condition` selector in Metadirective (PR #86457)

[Robin Caloudis via cfe-commits]

robincaloudis wrote:

>From what I understood, I'd say that there is a chance that wrong semantics are attached against the `trait-selector` when it is a dynamic condition selector, i.e. `condition(non_const_val > 0)`. I'd base my argumentation on the fact that static condition selector do work. @shiltian, what do you think? Any other idea?

Code where `selector-set` (i.e. `user={}`) is parsed: https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseOpenMP.cpp#L2649-L2657

Code where `selector`(i.e. `condition(non_const_val > 0)`) is parsed: https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseOpenMP.cpp#L1185-L1225

https://github.com/llvm/llvm-project/pull/86457