c++: mangling of lambdas in default args [PR91241]

c++: mangling of lambdas in default args [PR91241]

In this testcase, the parms remembered in LAMBDA_EXPR_EXTRA_SCOPE are no
longer the parms of the FUNCTION_DECL they have as their DECL_CONTEXT, so we
were mangling both lambdas as parm #0.  But since the parms are numbered
from right to left we don't need to need to find them in the FUNCTION_DECL,
we can measure their own DECL_CHAIN.

gcc/cp/ChangeLog:

	PR c++/91241
	* mangle.c (write_compact_number): Add sanity check.
	(write_local_name): Use list_length for parm number.

gcc/testsuite/ChangeLog:

	PR c++/91241
	* g++.dg/abi/lambda-defarg1.C: New test.