[OpenMP][OMPD] Load `libompd` with LLVM dynamic lib (#222455)

OMPD loads `libompd.so` through a small helper, but that helper still
uses `dlopen/dlsym` under the hood. We have already laid the foundation
by making libraries load behind helpers in the current state; this one
is simply a cleanup that moves helpers to LLVM's loader, which is LLVM's
standard.

The current state has few issues:

1: Still tied to Unix-style APIs.
⁠2: Windows support has to separately deal with loader differences.
3: Future shared-lib work would need more loader changes later.

This patch keeps the same OMPD API and call sites, but replaces the
implementation with LLVM's DynamicLibrary. Error handling remains
unchanged.

GitOrigin-RevId: 8eec838ca247c411c30b3452bc403b42e6d521e2