c++: Implement P2115R0 linkage changes for unnamed unscoped enums [PR120503]

c++: Implement P2115R0 linkage changes for unnamed unscoped enums [PR120503]

We currently list P2115R0 as implemented, but only the modules changes
had been done.  This patch implements the linkage changes so that
unnamed unscoped enums will use the name of the first enumerator for
linkage purposes.

This is (strictly speaking) a breaking change, as code that previously
relied on unnamed enumerations being internal linkage may have overloads
using those types become exposed and clash with other functions in a
different TU that have been similarly exposed.  As such this feature is
only implemented for C++20.

No ABI flag warning is provided, partly because C++20 is still an
experimental standard, but also because any affected functions could not
have been part of an ABI until this change anyway.

A number of testcases that are testing for behaviour of no-linkage types
are adjusted to use an enumeration with no values, so that the pre-C++20
and post-C++20 behaviour is equivalently tested.

In terms of implementation, I had originally considered adjusting the
DECL_NAME of the enum, as with 'name_unnamed_type', but this ended up
being more complicated as it had unwanted interactions with the existing
modules streaming and with name lookup and diagnostic messages.  This
patch instead uses a new function to derive this case.

The standard says that ([dcl.enum] p11) such an enum "...is denoted, for
linkage purposes, by its underlying type and its first enumerator", so
we need to add a new mangling production as well to handle this.

	PR c++/120503
	PR c++/120824

gcc/cp/ChangeLog:

	* cp-tree.h (TYPE_UNNAMED_P): Adjust for enums with enumerators
	for linkage purposes.
	(enum_with_enumerator_for_linkage_p): Declare.
	* decl.cc (name_unnamed_type): Adjust assertions to handle enums
	with enumerators for linkage purposes.
	(grokdeclarator): Use a typedef name for enums with enumerators
	for linkage purposes.
	(enum_with_enumerator_for_linkage_p): New function.
	(finish_enum_value_list): Reset type linkage for enums with
	enumerators for linkage purposes.
	* mangle.cc (write_unnamed_enum_name): New function.
	(write_unqualified_name): Handle enums with enumerators for
	linkage purposes.
	* tree.cc (decl_linkage): Fixup unnamed enums.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/mangle32.C: Remove enumerator list.
	* g++.dg/cpp0x/linkage2.C: Likewise.
	* g++.dg/ext/vector26.C: Likewise.
	* g++.dg/other/anon3.C: Likewise.
	* g++.dg/abi/mangle83.C: New test.
	* g++.dg/modules/enum-15_a.C: New test.
	* g++.dg/modules/enum-15_b.C: New test.

include/ChangeLog:

	* demangle.h (enum demangle_component_type): Add enumeration
	DEMANGLE_COMPONENT_UNNAMED_ENUM.

libiberty/ChangeLog:

	* cp-demangle.c (d_unnamed_enum): New function.
	(d_unqualified_name): Call it.
	(cplus_demangle_type): Handle unscoped unnamed types
	(Ue, Ul, etc.)
	(d_count_templates_scopes): Handle unnamed enums.
	(d_find_pack): Likewise.
	(d_print_comp_inner): Print unnamed enums.
	* testsuite/demangle-expected: Add tests.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>