[PATCH] D74806: [analyzer] NFCi: Refactor CStringChecker: use strongly typed internal API

[Balázs Benics via Phabricator via cfe-commits]

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CStringChecker is a huge beast.

My effort in improving the analyzer regarding taint analysis is humbled by multiple factors.
I wanted to extend the diagnostics of the CStringChecker with taintedness.

In the long run, the diagnostic emitting parts of the `GenericTaintChecker` would be migrated to multiple checkers, leaving it's responsibility only to *model* taint propagation.
Eg. the `GenericTaintChecker::checkTaintedBufferSize` functionality will be mostly part of the CStringChecker.

This plan 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.


Repository:
  rC Clang

https://reviews.llvm.org/D74806

Files:
  clang/lib/StaticAnalyzer/Checkers/CStringChecker.cpp
  clang/test/Analysis/bsd-string.c
  clang/test/Analysis/bstring.c
  clang/test/Analysis/null-deref-ps-region.c
  clang/test/Analysis/string.c

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