[clang][analyzer] Errno modeling code refactor (NFC).

Some of the code used in StdLibraryFunctionsChecker is applicable to
other checkers, this is put into common functions. Errno related
parts of the checker are simplified and renamed. Documentations in
errno_modeling functions are updated.

This change makes it available to have more checkers that perform
modeling of some standard functions. These can set the errno state
with common functions and the bug report messages (note tags) can
look similar.

Reviewed By: steakhal, martong

Differential Revision: https://reviews.llvm.org/D131879