[DWARFLibrary] Add support to re-construct cu-index

[DWARFLibrary] Add support to re-construct cu-index

According to DWARF5 specification and gnu specification for DWARF4 the offset
entry in the CU/TU Index is 32 bits. This presents a problem when
.debug_info.dwo in DWP file grows beyond 4GB. The CU Index becomes partially
corrupted.

This diff adds manual parsing of .debug_info.dwo/.debug_abbrev.dwo to
reconstruct CU index in general, and TU index for DWARF5. This is a work around
until DWARF6 spec is finalized.

Next patch will change internal CU/TU struct to 64 bit, and change uses as
necessary. The plan is to land all the patches in one go after all are approved.

This patch originates from the discussion in: https://discourse.llvm.org/t/dwarf-dwp-4gb-limit/63902

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D137882