c++: C++20 class NTTP trailing zero-init [PR100079]

The new testcase was breaking because constexpr evaluation was simplifying
Bar{Baz<42>{}} to Bar{empty}, but then we weren't treating them as
equivalent.  Poking at this revealed that the code for eliding trailing
zero-initialization in class non-type template argument mangling was pretty
broken, including the test, mangle71.

I dealt with the FIXME to support RANGE_EXPR, and fixed the confusion
between a list-initialized temporary mangled as written (i.e. in the
signature of a function template) and a template parameter object mangled as
the value representation of the object.  I'm distinguishing between these
using COMPOUND_LITERAL_P.  A later patch will adjust the use of
COMPOUND_LITERAL_P to be more useful for this distinction, but it works now
for distinguishing these cases in mangling.

gcc/cp/ChangeLog:

	PR c++/100079
	* cp-tree.h (first_field): Declare.
	* mangle.c (range_expr_nelts): New.
	(write_expression): Improve class NTTP mangling.
	* pt.c (get_template_parm_object): Clear TREE_HAS_CONSTRUCTOR.
	* tree.c (zero_init_expr_p): Improve class NTTP handling.
	* decl.c: Adjust comment.

gcc/testsuite/ChangeLog:

	PR c++/100079
	* g++.dg/abi/mangle71.C: Fix expected mangling.
	* g++.dg/abi/mangle77.C: New test.
	* g++.dg/cpp2a/nontype-class-union1.C: Likewise.
	* g++.dg/cpp2a/nontype-class-equiv1.C: Removed.
	* g++.dg/cpp2a/nontype-class44.C: New test.

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