c++: ICE with constexpr lambda [PR107280]

c++: ICE with constexpr lambda [PR107280]

We crash here since r10-3661, the store_init_value hunk in particular.
Before, we called cp_fully_fold_init, so e.g.

  {.str=VIEW_CONVERT_EXPR<char[8]>("")}

was folded into

  {.str=""}

but now we don't fold and keep the VCE around, and it causes trouble in
cxx_eval_store_expression: in the !refs->is_empty () loop we descend on
.str's initializer but since it's wrapped in a VCE, we skip the STRING_CST
check and then crash on the CONSTRUCTOR_NO_CLEARING.

	PR c++/107280

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_store_expression): Strip location wrappers.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/constexpr-lambda28.C: New test.