Don't set TUScope to null when generating a module in incremental processing mode.

Summary: When in incremental processing mode, we should never set `TUScope` to a nullptr otherwise any future lookups fail. We already have similar checks in the rest of the code, but we never hit this one because so far we didn't try to generate a module from the AST that Cling generates.

Reviewers: rsmith, v.g.vassilev

Reviewed By: v.g.vassilev

Subscribers: cfe-commits, v.g.vassilev

Differential Revision: https://reviews.llvm.org/D35536

llvm-svn: 308333