[CrossTU][NFCI] Refactor loadExternalAST function

Summary:
Refactor loadExternalAST method of CrossTranslationUnitContext in order to
reduce maintenance burden and so that features are easier to add in the future.

Reviewers: martong

Subscribers: rnkovacs, dkrupp, Szelethus, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64753

llvm-svn: 367829