c++: Fix reuse of class constants [PR94453]

c++: Fix reuse of class constants [PR94453]

The testcase hit an ICE trying to expand a TARGET_EXPR temporary cached from
the other lambda-expression.  This patch fixes this in two ways:

1) Avoid reusing a TARGET_EXPR from another function.
2) Avoid ending up with a TARGET_EXPR at all; the use of 'p' had become
<TARGET_EXPR<NON_LVALUE_EXPR<TARGET_EXPR ...>>>, which doesn't make any
sense.

gcc/cp/ChangeLog
2020-04-04  Jason Merrill  <jason@redhat.com>

	PR c++/94453
	* constexpr.c (maybe_constant_value): Use break_out_target_exprs.
	* expr.c (mark_use) [VIEW_CONVERT_EXPR]: Don't wrap a TARGET_EXPR in
	NON_LVALUE_EXPR.