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

lldb/test/Shell/SymbolFile/DWARF/Inputs/ModuleOwnership/module.modulemap

@@ -0,0 +1,6 @@
+module A {
+  header "A.h"
+  module B {
+    header "B.h"
+  }
+}