Prevent mixups of IDENTIFIER_TRANSPARENT_ALIAS and IDENTIFIER_INTERNAL_P better ...

Prevent mixups of IDENTIFIER_TRANSPARENT_ALIAS and IDENTIFIER_INTERNAL_P better [PR120855]

The assertion failure on ASM_OUTPUT_WEAKREF targets since my r16-1738
was caused because the 'TREE_CHAIN (id)' check in assemble_name_resolve
no longer implies that ID is a transparent alias, since internal
identifiers can have a TREE_CHAIN as well.

I still don't think it's possible for a transparent alias to be an
internal identifier in the sense added, so this patch simply constrains
the assertion better so that it doesn't fail spuriously.  I also added a
couple of other assertions to help validate this assumption.

	PR middle-end/120855

gcc/ChangeLog:

	* cgraphunit.cc (symbol_table::compile): Assert a transparent
	alias is not an internal identifier.
	* symtab.cc (symbol_table::change_decl_assembler_name):
	Likewise.
	* varasm.cc (assemble_name_resolve): Check for
	IDENTIFIER_TRANSPARENT_ALIAS instead of just TREE_CHAIN.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>