This implements ABI-compliant lambda discriminators.  Not only do we
have per-scope counters, but we also distinguish by lambda signature.
Only lambdas with the same signature will need non-zero
discriminators.  As the discriminator is signature-dependent, we have
to process the lambda function's declaration before we can determine
it.  For templated and generic lambdas the signature is that of the
uninstantiated lambda -- not separate for each instantiation.

With this change, gcc and clang now produce the same lambda manglings
for all these testcases.

nathan

-- 
Nathan Sidwell