Disable unnecessary calls to dwarf2out_decl for the parents of nested
functions. This fixes a libstdc++ build regression on
x86_64-apple-darwin, as these calls used to add duplicate attributes
(for instance DW_AT_inline and DW_AT_object_pointer) to the
corresponding DW_TAG_subprogram DIE.

Bootstrapped and regtested on x86_64-linux.

gcc/
	PR debug/78112
	* dwarf2out.c (dwarf2out_early_global_decl): Call dwarf2out_decl
	on the context only when it has no DIE yet.

testsuite/
	PR debug/78112
	* g++.dg/pr78112.C: New testcase

Co-Authored-By: Jakub Jelinek <jakub@redhat.com>

From-SVN: r242035