rs6000: Handle unresolved overloaded builtin [PR105485]

rs6000: Handle unresolved overloaded builtin [PR105485]

PR105485 exposes that new builtin function framework doesn't handle
unresolved overloaded builtin function well.  With new builtin
function support, we don't have builtin info for any overloaded
rs6000_gen_builtins enum, since they are expected to be resolved to
one specific instance.  So when function rs6000_gimple_fold_builtin
faces one unresolved overloaded builtin, the access for builtin info
becomes out of bound and gets ICE then.

We should not try to fold one unresolved overloaded builtin there
and as the previous support we should emit one error message during
expansion phase like "unresolved overload for builtin ...".

	PR target/105485

gcc/ChangeLog:

	* config/rs6000/rs6000-builtin.cc (rs6000_gimple_fold_builtin): Add
	the handling for unresolved overloaded builtin function.
	(rs6000_expand_builtin): Likewise.

gcc/testsuite/ChangeLog:

	* g++.target/powerpc/pr105485.C: New test.

(cherry picked from commit 94504c9ae157db937a2e62d533a36d56598f3c09)