c++: Fix FMV return type ambiguation

c++: Fix FMV return type ambiguation

Add logic for the case of two FMV annotated functions with identical
signature other than the return type.

Previously this was ignored, this changes the behavior to emit a diagnostic.

gcc/cp/ChangeLog:
	PR c++/119498
	* decl.cc (duplicate_decls): Change logic to not always exclude FMV
	annotated functions in cases of return type non-ambiguation.

gcc/testsuite/ChangeLog:
	PR c++/119498
	* g++.target/aarch64/pr119498.C: New test.