c++: Fix usage of CONSTRUCTOR_PLACEHOLDER_BOUNDARY inside array initializers [PR90996]

c++: Fix usage of CONSTRUCTOR_PLACEHOLDER_BOUNDARY inside array initializers [PR90996]

This PR reports that ever since the introduction of the
CONSTRUCTOR_PLACEHOLDER_BOUNDARY flag, we are sometimes failing to resolve
PLACEHOLDER_EXPRs inside array initializers that refer to some inner
constructor.  In the testcase in the PR, we have as the initializer for "S c[];"
the following

  {{.a=(int &) &_ZGR1c_, .b={*(&<PLACEHOLDER_EXPR struct S>)->a}}}

where CONSTRUCTOR_PLACEHOLDER_BOUNDARY is set on the middle constructor.  When
calling replace_placeholders from store_init_value, we pass the entire
initializer to it, and as a result we fail to resolve the PLACEHOLDER_EXPR
within due to the CONSTRUCTOR_PLACEHOLDER_BOUNDARY flag on the middle
constructor blocking replace_placeholders_r from reaching it.

To fix this, we could perhaps either call replace_placeholders in more places,
or we could change where we set CONSTRUCTOR_PLACEHOLDER_BOUNDARY.  This patch
takes this latter approach -- when building up an array initializer, we now
bubble any CONSTRUCTOR_PLACEHOLDER_BOUNDARY flag from the element initializers
up to the array initializer so that the boundary doesn't later impede us when we
call replace_placeholders from store_init_value.

Besides fixing the kind of code like in the testcase, this shouldn't cause any
other differences in PLACEHOLDER_EXPR resolution because we don't create or use
PLACEHOLDER_EXPRs of array type in the frontend, as far as I can tell.

gcc/cp/ChangeLog:

	PR c++/90996
	* tree.c (replace_placeholders): Look through all handled components,
	not just COMPONENT_REFs.
	* typeck2.c (process_init_constructor_array): Propagate
	CONSTRUCTOR_PLACEHOLDER_BOUNDARY up from each element initializer to
	the array initializer.

gcc/testsuite/ChangeLog:

	PR c++/90996
	* g++.dg/cpp1y/pr90996.C: New test.