[clang][analyzer] Fix StdLibraryFunctionsChecker 'mkdir' return value.

The functions 'mkdir', 'mknod', 'mkdirat', 'mknodat' return 0 on success
and -1 on failure. The checker modeled these functions with a >= 0
return value on success which is changed to 0 only. This fix makes
ErrnoChecker work better for these functions.

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D127277