[clang] [clang][analyzer] Add checker 'security.UnsafeSymlinkTest' (PR #221184)

[Balázs Kéri via cfe-commits]

balazske wrote:

I have changed many parts of the code and documentation and tried to fix all review comments.
The previous way of emitting messages was not always correct. Now the "S_ISLNK" check can happen before or after the open of file, and there is only a single type of error situation that occurs at "file access" (not at open).
Improvements are still possible, like indicate what `st_` fields are not compared, or check of write into the `st_` fields before comparison (in this case we can not be sure what is the intent and it is better to omit the warning).

https://github.com/llvm/llvm-project/pull/221184