Fix a bug in the .apple_exttypes recognition; a bug crept in as this method has been updated over the years so that we were not using that accelerator table when it was present.  Also change this like the other accelerator tables to make a distinction between "is valid" and "has content" -- a valid accelerator table with no content means that we should not look through the dwarf for lookups; there are none defined in this file.
Fix a bug in the .apple_exttypes recognition; a bug crept in as
this method has been updated over the years so that we were not
using that accelerator table when it was present.  Also change this
like the other accelerator tables to make a distinction between "is
valid" and "has content" -- a valid accelerator table with no content
means that we should not look through the dwarf for lookups; there
are none defined in this file.

<rdar://problem/30867462>

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb