You are not alone. We have the same warning:
LLVM ERROR: Error opening /Users/xxx/Library/Developer/Xcode/DerivedData/xxx/Build/Intermediates.noindex/ArchiveIntermediates/xxx/IntermediateBuildFilesPath/xxx.build/Release-iphoneos/xxx.build/MetalKernels.air': No such file or directory!
warning: linking module flags 'SDK Version': IDs have conflicting values
I have the same warning - no idea what it means.
The Metal code works, so there's that. Still annoying tho'.
I have the same warning iOS 13 XCode 11
Would love to know how to get to the bottom of it.