c++: mangle multiple levels of template parms [PR109422]

This becomes be more important with concepts, but can also be seen with
generic lambdas.

	PR c++/109422

gcc/cp/ChangeLog:

	* mangle.cc (write_template_param): Also mangle level.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-generic-mangle1.C: New test.
	* g++.dg/cpp2a/lambda-generic-mangle1a.C: New test.