see also https://bugs.swift.org/browse/TF-573
This linker issue was fixed in https://github.com/apple/swift/pull/25328.
A new macOS toolchain is being built now. I'll reply with a link to the new toolchain soon.
New June 12, 2019 Xcode toolchain posted under Development Snapshots.
Please note that we're experiencing code signing issue, so you will likely encounter:

To install the toolchain, go to System Preferences > Security & Privacy > Open Anyway.
@dan-zheng
I appreciate your fast response and care! it works great now and I can FINALLY jump to definitions!
Most helpful comment
New June 12, 2019 Xcode toolchain posted under Development Snapshots.
Please note that we're experiencing code signing issue, so you will likely encounter:

To install the toolchain, go to
System Preferences > Security & Privacy > Open Anyway.