[Flang][OpenMP] Prevent ICE for certain constructs in unnamed programs (#73938)

This patch fixes #72748 by modifying the processing of program units to
search for a symbol to which OpenMP REQUIRES clauses can bind to. Rather
than picking up the first PFT node with a source reference and getting
its associated scope, it picks up the last one.

This avoids using the source from the first specification construct of
a nameless program, which can sometimes not be associated to any scope,
causing an ICE due to an invalid source location.