[CodeGen] Use getCharWidth() more consistently in CGRecordLowering. NFC

When using getByteArrayType the requested size is calculated in
char units, but the type used for the array was hardcoded to the
Int8Ty. This patch is using getCharWIdth a bit more consistently
by using getIntNTy in combination with getCharWidth, instead
of explictly using getInt8Ty.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D94977