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).

llvm-svn: 181392