[clang] [clang][analyzer] Fix false positive in StdLibraryFunctionsChecker for mmap with MAP_ANON (PR #219568)

[Balázs Benics via cfe-commits]

https://github.com/steakhal requested changes to this pull request.

Looks great. Thanks!
I had some nits here and there but nothing ground breaking.

https://github.com/llvm/llvm-project/pull/219568