[clang][Sema] Use size of char in bits for void types

[clang][Sema] Use size of char in bits for void types

The extension that allows for pointer arithmetic on 'void' types treats
the 'void' as a 'char'. We should use the 'char' size in bits instead of
1 (the number of bytes) to allow warning when pointer arithmetic would
go out of bounds.

Differential Revision: https://reviews.llvm.org/D135989