[analyzer] ArrayBoundCheckerV2: suppress false positives from ctype macros

The checker alpha.security.ArrayBoundV2 created bug reports in
situations when the (tainted) result of fgetc() or getchar() was passed
to one of the isXXXXX() macros from ctype.h.

This is a common input handling pattern (within the limited toolbox of
the C language) and several open source projects contained code where it
led to false positive reports; so this commit suppresses ArrayBoundV2
reports generated within the isXXXXX() macros.

Note that here even true positive reports would be difficult to
understand, as they'd refer to the implementation details of these
macros.

Differential Revision: https://reviews.llvm.org/D149460