[DX] Improve parse error messages

This change refactors the parte parsing logic to operate on StringRefs
of the part data rather than starting from an offset and splicing down.
It also improves some of the error reporting around part layout.

Specifically, this code now reports a distinct error if there isn't
enough data in the buffer to store the part size and it reports an
error if the parts overlap.

Reviewed By: bob80905

Differential Revision: https://reviews.llvm.org/D139681