[Sema] Remove dead code in BuildAnonymousStructUnionMemberReference. NFCI

Summary:
This patch cleans up a bunch of dead or unused code in BuildAnonymousStructUnionMemberReference.

The dead code was a branch that built a new CXXThisExpr when we weren't given a base object expression or base variable.
However, BuildAnonymousFoo has only two callers. One of which always builds a base object expression first, the second only calls when the IndirectFieldDecl is not a C++ class member. Even within C this branch seems entirely unused.

I tried diligently to write a test which hit it with no success. 

This patch removes the branch and replaces it with an assertion that we were given either a base object expression or a base variable.


Reviewers: rsmith, aaron.ballman, majnemer, rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D45410

llvm-svn: 329518