[clang] [llvm] [NFC][analyzer] Extract bounds checking library (PR #202372)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

Ok, then I'll close this and prepare a new PR.

That way I will be able to cleanly apply the ideas that surfaced in this PR instead of trying to re-refactor the current state in those directions.

I will also probably create a small functional change before this "second attempt" patch to ensure that the "assuming in bounds" path notes and "out of bounds access" warnings use the same same element type in the diagnostics. (It has a very minor positive impact on the user experience + lets me eliminate `getAsCleanArraySubscriptExpr`.)

https://github.com/llvm/llvm-project/pull/202372