c++: Revert unnecessary parts of fix for [PR90996]

c++: Revert unnecessary parts of fix for [PR90996]

The process_init_constructor_array part of my PR90996 patch turns out to
be neither necessary nor sufficient to make the pr90996.C testcase work,
and I wasn't able to come up with a testcase that demonstrates this part
is ever necessary.

gcc/cp/ChangeLog:

	Revert:

	2020-04-07  Patrick Palka  <ppalka@redhat.com>

	PR c++/90996
	* 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: Turn into execution test to verify
	that each PLACEHOLDER_EXPR gets correctly resolved.