c++: Fix mangling of lambdas in static data member initializers [PR107741]

This fixes an issue where lambdas declared in the initializer of a
static data member within the class body do not get a mangling scope of
that variable; this results in mangled names that do not conform to the
ABI spec.

To do this, the patch splits up grokfield for this case specifically,
allowing a declaration to be build and used in start_lambda_scope before
parsing the initializer, so that record_lambda_scope works correctly.

As a drive-by, this also fixes the issue of a static member not being
visible within its own initializer.

	PR c++/107741

gcc/c-family/ChangeLog:

	* c-opts.cc (c_common_post_options): Bump ABI version.

gcc/ChangeLog:

	* common.opt: Add -fabi-version=20.
	* doc/invoke.texi: Likewise.

gcc/cp/ChangeLog:

	* cp-tree.h (start_initialized_static_member): Declare.
	(finish_initialized_static_member): Declare.
	* decl2.cc (start_initialized_static_member): New function.
	(finish_initialized_static_member): New function.
	* lambda.cc (record_lambda_scope): Support falling back to old
	ABI (maybe with warning).
	* parser.cc (cp_parser_member_declaration): Build decl early
	when parsing an initialized static data member.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/macro0.C: Bump ABI version.
	* g++.dg/abi/mangle74.C: Remove XFAILs.
	* g++.dg/other/fold1.C: Restore originally raised error.
	* g++.dg/abi/lambda-ctx2-19.C: New test.
	* g++.dg/abi/lambda-ctx2-19vs20.C: New test.
	* g++.dg/abi/lambda-ctx2-20.C: New test.
	* g++.dg/abi/lambda-ctx2.h: New test.
	* g++.dg/cpp0x/static-member-init-1.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>