c++: Member initializer list diagnostic locations [PR94024]

c++: Member initializer list diagnostic locations [PR94024]

This patch preserves the source locations of each node in a member
initializer list so that during processing of the list we can set
input_location appropriately for generally more accurate diagnostic
locations.  Since TREE_LIST nodes are tcc_exceptional, they can't have
source locations, so we instead store the location in a dummy
tcc_expression node within the TREE_TYPE of the list node.

gcc/cp/ChangeLog:

	PR c++/94024
	* init.c (sort_mem_initializers): Preserve TREE_TYPE of the
	member initializer list node.
	(emit_mem_initializers): Set input_location when performing each
	member initialization.
	* parser.c (cp_parser_mem_initializer): Attach the source
	location of this initializer to a dummy EMPTY_CLASS_EXPR
	within the TREE_TYPE of the list node.
	* pt.c (tsubst_initializer_list): Preserve TREE_TYPE of the
	member initializer list node.

gcc/testsuite/ChangeLog:

	PR c++/94024
	* g++.dg/diagnostic/mem-init1.C: New test.