[analyzer] StdLibraryFunctionsChecker: Add support for new functions

`toupper`, `tolower`, `toascii` functions were added to
StdLibraryFunctionsChecker to fully cover CERT STR37-C rule:
https://wiki.sei.cmu.edu/confluence/x/BNcxBQ

Differential Revision: https://reviews.llvm.org/D85093