debug/96690 - mangle symbols eventually used by late dwarf output

debug/96690 - mangle symbols eventually used by late dwarf output

The following makes sure to, at early debug generation time, mangle
symbols we eventually end up outputting during late finish.

2020-08-24  Richard Biener  <rguenther@suse.de>

	PR debug/96690
	* dwarf2out.c (reference_to_unused): Make FUNCTION_DECL
	processing more consistent with respect to
	symtab->global_info_ready.
	(tree_add_const_value_attribute): Unconditionally call
	rtl_for_decl_init to do all mangling early but throw
	away the result if early_dwarf.

	* g++.dg/lto/pr96690_0.C: New testcase.