Add testing for DW_OP_piece and fix a bug with small Scalar values.

By switching to Scalars that are backed by explicitly-sized APInts we
can avoid a bug that increases the buffer reserved for a small piece
to the next-largest host integer type.

This manifests as "DW_OP_piece for offset foo but top of stack is of size bar".

Differential Revision: https://reviews.llvm.org/D72879