Reland "[lldb][DWARFASTParserClang] Correctly resolve imported namespaces...

Reland "[lldb][DWARFASTParserClang] Correctly resolve imported namespaces during expression evaluation"

This relands a patch previously reverted
in `181d6e24`.
This wasn't quite working on Linux because we
weren't populating the manual DWARF index with
`DW_TAG_imported_declaration`. The relanded patch
does this.

**Summary**

This patch makes the expression evaluator understand
namespace aliases.

This will become important once `std::ranges` become
more widespread since `std::views` is defined as:

```
namespace std {
namespace ranges::views {}

namespace views = ranges::views;
}
```

**Testing**

* Added API test

Differential Revision: https://reviews.llvm.org/D143398