[DebugInfo][Support] Replace DWARFDataExtractor size function

This patch adds a new size function to the base DataExtractor class,
which removes the need for the DWARFDataExtractor size function.

It is unclear why DWARFDataExtractor's size function returned zero in
some circumstances (i.e. when it is constructed without a section, and
with a different data source instead), so that behaviour has changed.
The old behaviour could cause an assertion in the debug line parser, as
the size did not reflect the actual data available, and could be lower
than the current offset being parsed.

Reviewed by: dblaikie

Differential Revision: https://reviews.llvm.org/D72337