[Sema] Refactor LookupVisibleDecls. NFC

Summary:
We accumulated some configuration parameters for LookupVisibleDecls that
are being passed unchanged to recursive calls, e.g. LoadExternal and
IncludeDependentBases.

At the same time, there is a bunch of parameters that can change in the
recursive invocations.

It is hard to tell the difference between those groups, making the code
hard to follow.

This change introduces a helper struct and factors out the non-changing
bits into fields, making recursive calls in the implementation code easier
to read.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: riccibruno, doug.gregor, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65752

llvm-svn: 371032