[Object] Refactor build ID parsing into Object lib.

[Object] Refactor build ID parsing into Object lib.

This makes parsing for build IDs in the markup filter slightly more
permissive, in line with fromHex.

It also removes the distinction between missing build ID and empty build
ID; empty build IDs aren't a useful concept, since their purpose is to
uniquely identify a binary. This removes a layer of indirection wherever
build IDs are obtained.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D147485