[analyzer] NFCi: Refactor CStringChecker: use strongly typed internal API

Summary:
I wanted to extend the diagnostics of the CStringChecker with taintedness.
This requires the CStringChecker to be refactored to support a more flexible
reporting mechanism.

This patch does only refactorings, such:
 - eliminates always false parameters (like WarnAboutSize)
 - reduces the number of parameters
 - makes strong types differentiating *source* and *destination* buffers
   (same with size expressions)
 - binds the argument expression and the index, making diagnostics accurate
   and easy to emit
 - removes a bunch of default parameters to make it more readable
 - remove random const char* warning message parameters, making clear where
   and what is going to be emitted

Note that:
 - CheckBufferAccess now checks *only* one buffer, this removed about 100 LOC
   code duplication
 - not every function was refactored to use the /new/ strongly typed API, since
   the CString related functions are really closely coupled monolithic beasts,
   I will refactor them separately
 - all tests are preserved and passing; only the message changed at some places.
   In my opinion, these messages are holding the same information.

I would also highlight that this refactoring caught a bug in
clang/test/Analysis/string.c:454 where the diagnostic did not reflect reality.
This catch backs my effort on simplifying this monolithic CStringChecker.

Reviewers: NoQ, baloghadamsoftware, Szelethus, rengolin, Charusso

Reviewed By: NoQ

Subscribers: whisperity, xazax.hun, szepet, rnkovacs, a.sidorin,
mikhail.ramalho, donat.nagy, dkrupp, Charusso, martong, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74806