Preserve the owning module information from DWARF in the synthesized AST

Types that came from a Clang module are nested in DW_TAG_module tags
in DWARF. This patch recreates the Clang module hierarchy in LLDB and
sets the owning module information accordingly. My primary motivation
is to facilitate looking up per-module APINotes for individual
declarations, but this likely also has other applications.

rdar://problem/59634380

Differential Revision: https://reviews.llvm.org/D75488