lldb failure with Swift 4.0 on Ubuntu 16.04

I installed the June 7th Ubuntu 16.04 release and performed a clean build
with no problems.

However when I try to debug I get the following error, and then LLDB

const swift::Lowering::TypeLowering
Assertion `key.SubstType->isLegalSILType() && "type is not already
lowered"' failed.*

I installed the June 9th snapshot and it has the same problem. I tested
both Swift 3.1.1 and Swift 4.0 modes, but it fails the same way for both.

Anyone else see this problem?? Any ideas??

Thanks, Ed