[DebugInfo] Change DWARFDebugAbbrev initialization

I plan on adding better error handling to DWARFDebugAbbrev, but first I
want us to be able to better reason about a state of a DWARFDebugAbbrev.
I want us to be able to initialize a DWARFDebugAbbrev in its complete
state instead of creating it and then calling an `extract` method
manually. If its Data field is populated, then parsing is not complete.
If Data is `std::nullopt`, then parsing is done and this section is
"finalized" in some sense.

Additionally, I have removed the `clear()` method. This makes sense for other
classes like DWARFAbbreviationDeclaration where we may want to re-use an object
repeatedly to avoid repeated initializations and allocations, but for
DWARFDebugAbbrev we pretty much create one and stick with it.

Differential Revision: https://reviews.llvm.org/D152947