c++: unset COMPOUND_LITERAL_P [PR100079]

c++: unset COMPOUND_LITERAL_P [PR100079]

Once a CONSTRUCTOR has been digested and used as an initializer, it no
longer represents a compound literal by itself, so we can clear the flag,
letting us use it consistently to distinguish between digested and
undigested initializer-lists.

gcc/cp/ChangeLog:

	* cp-tree.h: Clarify comments.
	* pt.c (get_template_parm_object): Add assert.
	* semantics.c (finish_compound_literal): Clear TREE_HAS_CONSTRUCTOR.
	* tree.c (zero_init_expr_p): Check TREE_HAS_CONSTRUCTOR.
	* typeck2.c (store_init_value): Likewise.