Handle errors in lambda prototype instantiation correctly

Previously we would attempt to build a TypeSourceInfo for a null type,
and then we would forget to pop the function scope before returning an
error.

Reviewers: rsmith

Differential Revision: http://reviews.llvm.org/D6665

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224271 91177308-0d34-0410-b5e6-96231b3b80d8