うーん、Xcodeのテスト実行からだと Foundation.Process で /usr/bin/swiftc を実行しようとすると、

2021-06-05 15:20:12.963749+0900 swiftc[99910:13408271] Failed to open macho file at /Applications/Xcode12.5.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc for reading: Too many levels of symbolic links