Fix an edge case of mangling involving the combination of a lambda and typeid.

typeid (and a couple other non-standard places where we can transform an
unevaluated expression into an evaluated expression) is special
because it introduces an an expression evaluation context,
which conflicts with the mechanism to compute the current
lambda mangling context.  PR12123.

I would appreciate if someone would double-check that we get the mangling
correct with this patch.

llvm-svn: 164658