Fix inliner ICE on alias with flatten attribute [PR92372]

Fix inliner ICE on alias with flatten attribute [PR92372]

gcc/ChangeLog:

2020-03-19  Jan Hubicka  <hubicka@ucw.cz>

	PR ipa/92372
	* cgraphunit.c (process_function_and_variable_attributes): warn
	for flatten attribute on alias.
	* ipa-inline.c (ipa_inline): Do not ICE on flatten attribute on alias.

gcc/testsuite/ChangeLog:

2020-03-19  Jan Hubicka  <hubicka@ucw.cz>

	PR ipa/92372
	* gcc.c-torture/pr92372.c: New test.
	* gcc.dg/attr-flatten-1.c: New test.