FoundationXML import issue in 5.3.0 release

[MaxDesiatov]

🎉 Working great so far.

The only minor surprise was this (using the Ubuntu 20 toolchain):

error: missing required module 'CFXMLInterface'
  import FoundationXML
         ^

That import is guarded by #if canImport(FoundationXML), and used to be skipped safely. So it seems FoundationXML is half there and half missing. (Adding && !os(WASI) worked just fine.)

Originally posted by @SDGGiesbrecht in #1759 (comment)