[include-fixer] A refactoring of IncludeFixerContext.

Summary:
No functional changes in this patch. It is a refactoring (pull out a
structure representing the symbol being queried).

This is a preparing step for inserting missing namespace qualifiers to all
instances of an unidentified symbol.

Reviewers: bkramer

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D22510

llvm-svn: 275980