DWARF: Introduce DWARFTypeUnit class

Summary:
This patch introduces the DWARFTypeUnit class, and teaches lldb to parse
type units out of both the debug_types section (DWARF v4), and from the
regular debug_info section (DWARF v5).

The most important piece of functionality - resolving DW_AT_signatures
to connect type forward declarations to their definitions - is not
implemented here, but even without that, a lot of functionality becomes
available. I've added tests for the commands that start to work after
this patch.

The changes in this patch were greatly inspired by D61505, which in turn took
over changes from D32167.

Reviewers: JDevlieghere, clayborg, aprantl

Subscribers: mgorny, jankratochvil, lldb-commits

Differential Revision: https://reviews.llvm.org/D62008

llvm-svn: 361360