[OpenMP][NFC] Reuse OMPIRBuilder `struct ident_t` handling in Clang

[OpenMP][NFC] Reuse OMPIRBuilder `struct ident_t` handling in Clang

Replace the `ident_t` handling in Clang with the methods offered by the
OMPIRBuilder. This cuts down on the clang code as well as the
differences between the two, making further transitions easier. Tests
have changed but there should not be a real functional change. The most
interesting difference is probably that we stop generating local ident_t
allocations for now and just use globals. Given that this happens only
with debug info, the location part of the `ident_t` is probably bigger
than the test anyway. As the location part is already a global, we can
avoid the allocation, memcpy, and store in favor of a constant global
that is slightly bigger. This can be revisited if there are
complications.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D80735