c++: Only reject reinterpret casts from pointers to integers for manifestly_const_eva...

c++: Only reject reinterpret casts from pointers to integers for manifestly_const_eval evaluation [PR99456]

My PR82304/PR95307 fix moved reinterpret cast from pointer to integer
diagnostics from cxx_eval_outermost_constant_expr where it caught
invalid code only at the outermost level down into
cxx_eval_constant_expression.
Unfortunately, it regressed following testcase, we emit worse code
including dynamic initialization of some vars.
While the initializers are not constant expressions due to the
reinterpret_cast in there, there is no reason not to fold them as an
optimization.

I've tried to make this dependent on !ctx->quiet, but that regressed
two further tests, and on ctx->strict, which regressed other tests,
so this patch bases that on manifestly_const_eval.

The new testcase is now optimized as much as it used to be in GCC 10
and the only regression it causes is an extra -Wnarrowing warning
on vla22.C test on invalid code (which the patch adjusts).

2021-03-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99456
	* constexpr.c (cxx_eval_constant_expression): For CONVERT_EXPR from
	INDIRECT_TYPE_P to ARITHMETIC_TYPE_P, when !ctx->manifestly_const_eval
	don't diagnose it, set *non_constant_p nor return t.

	* g++.dg/opt/pr99456.C: New test.
	* g++.dg/ext/vla22.C: Expect a -Wnarrowing warning for c++11 and
	later.