c++: Templated lambda mangling

(Explicitly) Templated lambdas have a different signature to
implicitly templated lambdas -- '[]<template T> (T) {}' is not the
same as '[](auto) {}'.  This should be reflected in the mangling.  The
ABI captures this as
https://github.com/itanium-cxx-abi/cxx-abi/issues/31, and clang has
implemented such additions.

It's relatively straight forwards to write out the non-synthetic
template parms, and note if we need to issue an ABI warning.

	gcc/cp/
	* mangle.cc (write_closure_template_head): New.
	(write_closure_type_name): Call it.
	gcc/testsuite/
	* g++.dg/abi/lambda-ctx1-18.C: Adjust.
	* g++.dg/abi/lambda-ctx1-18vs17.C: Adjust.
	* g++.dg/abi/lambda-tpl1-17.C: New.
	* g++.dg/abi/lambda-tpl1-18.C: New.
	* g++.dg/abi/lambda-tpl1-18vs17.C: New.
	* g++.dg/abi/lambda-tpl1.h: New.