Fix cgraph_node::function_symbol availability compuattion [PR94202]

Fix cgraph_node::function_symbol availability compuattion [PR94202]

this fixes ICE in inliner cache sanity check which is caused by very old
bug in visibility calculation in cgraph_node::function_symbol and
cgraph_node::function_or_virtual_thunk_symbol.

In the testcase there is indirect call to a thunk. At begining we correctly
see that its body as AVAIL_AVAILABLE but later we inline into the thunk and
this turns it to AVAIL_INTERPOSABLE.

This is because function_symbol incorrectly overwrites availability parameter
by availability of the alias used in the call within thunk, which is a local
alias.

gcc/ChangeLog:

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

	PR ipa/94202
	* cgraph.c (cgraph_node::function_symbol): Fix availability computation.
	(cgraph_node::function_or_virtual_thunk_symbol): Likewise.

gcc/testsuite/ChangeLog:

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

	PR ipa/94202
	* g++.dg/torture/pr94202.C: New test.