Finish off mangling locals in block literals.

Specifically, handle the case where the block is in a default argument
in a class method.  The mangling here follows what we do for lambdas.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185991 91177308-0d34-0410-b5e6-96231b3b80d8