I figured it out. Although XC 11.3.1 can run on 10.14.6, the SDK is 10.15.
I went back to XC 10.3, which was the last version with the 10.14 SDK.
This gives Apple Swift version 5.0.1
and Apple LLVM version 10.0.1 (clang-1001.0.46.4)
Now I wonder if I can use SwiftUI with Swift 5.0.1 in XC 10.3 on 10.14.6 Mojave? "Outlook not so good." says Magic 8 Ball. :-)
Thank you xcodereleases.com; I should have looked more carefully before.
Sorry for the noise.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…