c++: Fix ICE with __builtin_bit_cast [PR98469]

c++: Fix ICE with __builtin_bit_cast [PR98469]

On the following testcase we ICE during constexpr evaluation (for warnings),
because the IL has ADDR_EXPR of BIT_CAST_EXPR and ADDR_EXPR case asserts
the result is not a CONSTRUCTOR.
The patch punts on lval BIT_CAST_EXPR folding.

> This change is OK, but part of the problem is that we're trying to do
> overload resolution for an S copy/move constructor, which we shouldn't be
> because bit_cast is a prvalue, so in C++17 and up we should use it to
> directly initialize the target without any implied constructor call.

This version therefore wraps it into a TARGET_EXPR then, it alone fixes
the bug, but I've kept the constexpr.c change too.

2021-01-05  Jakub Jelinek  <jakub@redhat.com>

	PR c++/98469
	* constexpr.c (cxx_eval_constant_expression) <case BIT_CAST_EXPR>:
	Punt if lval is true.
	* semantics.c (cp_build_bit_cast): Call get_target_expr_sfinae on
	the result if it has a class type.

	* g++.dg/cpp2a/bit-cast8.C: New test.
	* g++.dg/cpp2a/bit-cast9.C: New test.