[analyzer][StdLibraryFunctionsChecker] Elaborate the summary of fread and fwrite

Add the BufferSize argument constraint to fread and fwrite. This change
itself makes it possible to discover a security critical case, described
in SEI-CERT ARR38-C.

We also add the not-null constraint on the 3rd arguments.

In this patch, I also remove those lambdas that don't take any
parameters (Fwrite, Fread, Getc), thus making the code better
structured.

Differential Revision: https://reviews.llvm.org/D87081