[analyzer] On-demand parsing capability for CTU

Summary:
Introduce on-demand parsing of needed ASTs during CTU analysis.
The index-file format is extended, and analyzer-option CTUInvocationList
is added to specify the exact invocations needed to parse the needed
source-files.

Reviewers: martong, balazske, Szelethus, xazax.hun, whisperity

Reviewed By: martong, xazax.hun

Subscribers: gribozavr2, thakis, ASDenysPetrov, ormris, mgorny, whisperity, xazax.hun, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, Charusso, steakhal, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75665