[MS ABI] Don't crash while mangling recursive lambdas

We might get into bad situations where we try to embed the signature of
an inner lambda into an outer lambda which cannot work: the inner lambda
wants to embed the name of the outer lambda!

Instead, omit the return type for lambdas.

This fixes PR26105.

N.B.  While we are here, make lambdas nested within functions use an
artificial scope so that they can get demangled.

llvm-svn: 258003