[CodeComplete] Fix crash when completing params function declarations.

Summary:
In a decl like `int AA(BB cc)` where BB isn't defined, we end up trying to
parse `BB cc` as an expression (vexing parse) and end up triggering the
parser's "recovery-in-function" completion with no actual function
scope.

This patch avoids the assumption that such a scope exists in this context.

Reviewers: kadircet

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D53070

llvm-svn: 344133