**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
