[AST] Get field size in chars rather than bits in RecordLayoutBuilder.

In D79719, LayoutField was refactored to fetch the size of field
types in bits and then convert to chars, rather than fetching
them in chars directly. This is not ideal, since it makes the
calculations char size dependent, and breaks for sizes that
are not a multiple of the char size.

This patch changes it to use getTypeInfoInChars instead of
getTypeInfo.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D85191