c++: Fix up CONSTRUCTOR_PLACEHOLDER_BOUNDARY handling [PR105256]

c++: Fix up CONSTRUCTOR_PLACEHOLDER_BOUNDARY handling [PR105256]

The CONSTRUCTOR_PLACEHOLDER_BOUNDARY bit is supposed to separate
PLACEHOLDER_EXPRs that should be replaced by one object or subobjects of it
(variable, TARGET_EXPR slot, ...) from other PLACEHOLDER_EXPRs that should
be replaced by different objects or subobjects.
The bit is set when finding PLACEHOLDER_EXPRs inside of a CONSTRUCTOR, not
looking into nested CONSTRUCTOR_PLACEHOLDER_BOUNDARY ctors, and we prevent
elision of TARGET_EXPRs (through TARGET_EXPR_NO_ELIDE) whose initializer
is a CONSTRUCTOR_PLACEHOLDER_BOUNDARY ctor.  The following testcase ICEs
though, we don't replace the placeholders in there at all, because
CONSTRUCTOR_PLACEHOLDER_BOUNDARY isn't set on the TARGET_EXPR_INITIAL
ctor, but on a ctor nested in such a ctor.  replace_placeholders should be
run on the whole TARGET_EXPR slot.

So, the following patch fixes it by moving the CONSTRUCTOR_PLACEHOLDER_BOUNDARY
bit from nested CONSTRUCTORs to the CONSTRUCTOR containing those (but only
if it is closely nested, if there is some other tree sandwiched in between,
it doesn't do it).

2022-04-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/105256
	* typeck2.cc (process_init_constructor_array,
	process_init_constructor_record, process_init_constructor_union): Move
	CONSTRUCTOR_PLACEHOLDER_BOUNDARY flag from CONSTRUCTOR elements to the
	containing CONSTRUCTOR.

	* g++.dg/cpp0x/pr105256.C: New test.