Safer casting in ClangExpressionParser code completion

Summary:
Makes the code a bit safer in the unlikely situation that we don't get a ClangUserExpression
when doing code completion.

Reviewers: aprantl, jingham

Reviewed By: aprantl

Subscribers: labath, jdoerfert, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D59359

llvm-svn: 356174