Xcode: Use explicitFileType to mark source types (#14093)

Replace use of lastKnownFileType with explicitFileType to insist
that Xcode treat the file as we ask.
make CUDA_COMPUTE_BUILD_PATH use relative paths to binary dir.

make CUDA_COMPUTE_BUILD_PATH use a relative path to the current
binary directory instead of the current source directory if the source
file considered is in the current binary directory. This is done to
shorten the paths given to the compiler.
Merge topic 'xcode-explicitFileType' into next

5683146 Xcode: Use explicitFileType to mark source types (#14093)
Merge topic 'cuda_compute_build_path' into next

2d8355b make CUDA_COMPUTE_BUILD_PATH use relative paths to binary dir.