[lldb] Consider TAG_imported_declaration in DebugNamesIndex

In order to recognize namespace aliases as a namespace, the
DW_TAG_imported_declaration has to be taken into account. The name of these DIEs
is already included in all accelerator tables as of D143397.

Two of the three Index classes already handle this correctly:

1. ManualDWARFIndex (as of D143398)
2. AppleDWARFIndex works by default with D143397, since apple has a table
dedicated to namespaces.

This commit updates the third index class, DWARF 5's DebugNamesDWARFIndex.
As a result, it fixes the following test with DWARF 5:
commands/expression/namespace-alias/TestInlineNamespaceAlias.py

Differential Revision: https://reviews.llvm.org/D154730