[lldb][NFCI] Move functionality for getting unsupported DW_FORM values (#67579)

The LLVM implementation of DWARFDebugAbbrev does not have a way of
listing all the DW_FORM values that have been parsed but are unsupported
or otherwise unknown. AFAICT this functionality does not exist in LLVM
at all. Since my primary goal is to unify the implementations and not
judge the usefulness or completeness of this functionality, I decided to
move it out of LLDB's implementation of DWARFDebugAbbrev for the time
being.