[llvm][DebugInfo] Add DW_TAG_imported_declaration to accelerator tables

**Summary**

After this patch, `DW_TAG_imported_declaration`s will be emitted into
the DWARF accelerator tables (under `.apple_namespaces`)

**Motivation**

Currently LLDB expression evaluation doesn't see through namespace
aliases. This is because LLDB only considers namespaces that are
part of `.apple_namespaces` when building a nested namespace
identifier for C++, which currently doesn't include import
declarations. The alternative to putting imports into accelerator
tables is to do a linear scan of a `DW_TAG_namespace` and look
for import declarations that look like they would satisfy the lookup
request, which is prohibitively expensive.

**Testing**

* Added unit-test

Differential Revision: https://reviews.llvm.org/D143397