c++: Lambda context mangling

VAR and FIELD decls can become part of a lambda context, when the
lambda is 'attached' to that entity (It's a C++20 ODR thing that was
discovered with modules, but is actually separate.)  We were not
marking those decls as substitution candidates, leading to demangling
failures and variance from other compilers.

This patch bumps the ABI, and adds the contexts them to the
substitution table.  This is the intent of the ABI.

	gcc/
	* common.opt (-fabi-version=): Document 18.
	* doc/invoke.texi (-fabi-version): Document 18.
	gcc/c-family/
	* c-opts.cc (c_common_post_options): Bump abi to 18.
	gcc/cp/
	* mangle.cc (write_prefix): Add VAR_DECL & FIELD_DECL to
	substitution table under abi=18.  Note possible mismatch.
	gcc/testsuite/
	* g++.dg/abi/lambda-ctx1-17.C: New.
	* g++.dg/abi/lambda-ctx1-18.C: New.
	* g++.dg/abi/lambda-ctx1-18vs17.C: New.
	* g++.dg/abi/lambda-ctx1.h: New.
	* g++.dg/abi/lambda-vis.C: Adjust expected mangles.
	* g++.dg/abi/macro0.C: Adjust.

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