c++: Don't prune constant capture proxies only used in array dimensions [PR114292]

c++: Don't prune constant capture proxies only used in array dimensions [PR114292]

We currently ICE upon the following valid (under -Wno-vla) code

=== cut here ===
void f(int c) {
  constexpr int r = 4;
  [&](auto) { int t[r * c]; }(0);
}
=== cut here ===

When parsing the lambda body, and more specifically the multiplication,
we mark the lambda as LAMBDA_EXPR_CAPTURE_OPTIMIZED, which indicates to
prune_lambda_captures that it might be possible to optimize out some
captures.

The problem is that prune_lambda_captures then misses the use of the r
capture (because neither walk_tree_1 nor cp_walk_subtrees walks the
dimensions of array types - here "r * c"), hence believes the capture
can be pruned... and we trip on an assert when instantiating the lambda.

This patch changes cp_walk_subtrees so that (1) when walking a
DECL_EXPR, it also walks the DECL's type, and (2) when walking an
INTEGER_TYPE, it also walks its TYPE_{MIN,MAX}_VALUE. Note that #2 makes
a <case INTEGER_TYPE> redundant in for_each_template_parm_r, and removes
it.

	PR c++/114292

gcc/cp/ChangeLog:

	* pt.cc (for_each_template_parm_r) <INTEGER_TYPE>: Remove case
	now handled by cp_walk_subtrees.
	* tree.cc (cp_walk_subtrees): Walk the type of DECL_EXPR
	declarations, as well as the TYPE_{MIN,MAX}_VALUE of
	INTEGER_TYPEs.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/lambda-ice4.C: New test.