re PR middle-end/52640 (performance bottleneck: gcc/tree.c;value_member)

	PR middle-end/52640
	* varasm.c (pending_assemble_externals_set): New pointer set.
	(process_pending_assemble_externals): Destroy the pointer set.
	(assemble_external): See if decl is in pending_assemble_externals_set,
	and add it to pending_assemble_externals if necessary.
	(init_varasm_once): Allocate pending_assemble_externals_set.

	* gcc.c-torture/compile/limits-externdecl.c: New test.

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

From-SVN: r194441