c++: FIX_TRUNC_EXPR in tsubst [PR102990]

c++: FIX_TRUNC_EXPR in tsubst [PR102990]

This is a crash where a FIX_TRUNC_EXPR gets into tsubst_copy_and_build
where it hits gcc_unreachable ().

The history of tsubst_copy_and_build/FIX_TRUNC_EXPR is such that it
was introduced in r181478, but it did the wrong thing, whereupon it
was turned into gcc_unreachable () in r258821 (see this thread:
<https://gcc.gnu.org/pipermail/gcc-patches/2018-March/495853.html>).

In a template, we should never create a FIX_TRUNC_EXPR (that's what
conv_unsafe_in_template_p is for).  But in this test we are NOT in
a template when we call digest_nsdmi_init which ends up calling
convert_like, converting 1.0e+0 to int, so convert_to_integer_1
gives us a FIX_TRUNC_EXPR.

But then when we get to parsing f's parameters, we are in a template
when processing decltype(Helpers{}), and since r268321, when the
compound literal isn't instantiation-dependent and the type isn't
type-dependent, finish_compound_literal falls back to the normal
processing, so it calls digest_init, which does fold_non_dependent_init
and since the FIX_TRUNC_EXPR isn't dependent, we instantiate and
therefore crash in tsubst_copy_and_build.

The fateful call to fold_non_dependent_init comes from massage_init_elt,
We shouldn't be calling f_n_d_i on the result of get_nsdmi.  This we can
avoid by eschewing calling f_n_d_i on CONSTRUCTORs; their elements have
already been folded.

	PR c++/102990

gcc/cp/ChangeLog:

	* typeck2.cc (massage_init_elt): Avoid folding CONSTRUCTORs.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/nsdmi-template22.C: New test.
	* g++.dg/cpp0x/nsdmi-template23.C: New test.