[analyzer] StdLibraryFunctionsChecker: Use platform dependent EOF and UCharMax

Summary:
Both EOF and the max value of unsigned char is platform dependent. In this
patch we try our best to deduce the value of EOF from the Preprocessor,
if we can't we fall back to -1.

Reviewers: Szelethus, NoQ

Subscribers: whisperity, xazax.hun, kristof.beyls, baloghadamsoftware, szepet, rnkovacs, a.sidorin, mikhail.ramalh

Tags: #clang

Differential Revision: https://reviews.llvm.org/D74473