[clang][analyzer] Make messages of StdCLibraryFunctionsChecker user-friendly

Warnings and notes of checker alpha.unix.StdLibraryFunctionArgs are
improved. Previously one warning and one note was emitted for every
finding, now one warning is emitted only that contains a detailed
description of the found issue.

Reviewed By: Szelethus

Differential Revision: https://reviews.llvm.org/D143194