[clang] [clang][analyzer] Bring checker 'alpha.unix.cstring.NotNullTerminated' out of alpha (PR #113899)

[Balázs Kéri via cfe-commits]

balazske wrote:

I have added a FIXME to the place where no warning should appear. Additionally a new test file is added where only this checker is turned on (not the full `cstring`).

https://github.com/llvm/llvm-project/pull/113899