Add support for partial jump scope checking

This lets us diagnose and perform more complete semantic analysis when faced
with errors in the function body or declaration.

By recovering here we provide more consistent diagnostics, particularly during
interactive editing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208394 91177308-0d34-0410-b5e6-96231b3b80d8