[analyzer] NFC: Forbid array elements of void type.

Represent the symbolic value for results of pointer arithmetic on void pointers
in a different way: instead of making void-typed element regions, make
char-typed element regions.

Add an assertion that ensures that no void-typed regions are ever constructed.

This is a refactoring of internals that should not immediately affect
the analyzer's (default) behavior.

Differential Revision: https://reviews.llvm.org/D40939

llvm-svn: 322775