Finish renaming constructImportedModuleDIE to constructImportedEntityDIE
Debug Info: Support DW_TAG_imported_declaration

This provides basic functionality for imported declarations. For
subprograms and types some amount of lazy construction is supported (so
the definition of a function can proceed the using declaration), but it
still doesn't handle declared-but-not-defined functions (since we don't
generally emit function declarations).

Variable support is really rudimentary at the moment - simply looking up
the existing definition with no support for out of order (declaration,
imported_module, then definition).
Debug Info: Using declarations/DW_TAG_imported_declaration of variables, types, and functions.

Basic support is implemented here - it still doesn't account for
declared-but-not-defined variables or functions. It cannot handle out of
order (declared, 'using', then defined) cases for variables, but can
handle that for functions (& can handle declared, 'using'd, and not
defined at all cases for types).