Introduce DWARFDataExtractor::getInitialLength

Summary:
This patch introduces a function to house the code needed to do the
DWARF64 detection dance. The function decodes the initial length field
and returns it as a pair containing the actual length, and the DWARF
encoding.

This patch does _not_ attempt to handle the problem of detecting lengths
which extend past the size of the section, or cases when reads of a
single contribution accidentally escape beyond its specified length, but
I think it's useful in its own right.

Reviewers: dblaikie, jhenderson, ikudrin

Subscribers: hiraditya, probinson, aprantl, JDevlieghere, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D74560