[CGExpr] Use getCharWidth() more consistently in CCGExprConstant. NFC

Most of CGExprConstant.cpp is using the CharUnits abstraction
and is using getCharWidth() (directly of indirectly) when converting
between size of a char and size in bits. This patch is making that
abstraction more consistent by adding CharTy to the CodeGenTypeCache
(honoring getCharWidth() when mapping from char to LLVM IR types,
instead of using Int8Ty directly).

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D94979