OpenMP: Handle cpp_implicit_alias in declare-target discovery (PR96390)

OpenMP: Handle cpp_implicit_alias in declare-target discovery (PR96390)

Backport mainline version; updates commit
ef509d1985aa53a8c0875c25ad4050ea807be10e for later review-comment
changes.

gcc/ChangeLog:

	PR middle-end/96390
	* omp-offload.c (omp_discover_declare_target_tgt_fn_r): Handle
	alias nodes.

libgomp/ChangeLog:

	PR middle-end/96390
	* testsuite/libgomp.c++/pr96390.C: New test.
	* testsuite/libgomp.c-c++-common/pr96390.c: New test.

(cherry picked from commit 2a10a2c0689db280ee3a94164504b7196b8370f4)