[CMake] Use CMAKE_CURRENT_SOURCE_DIR as base for includes in SwiftCompilerSources
[CMake] Use CMAKE_CURRENT_SOURCE_DIR as base for includes in SwiftCompilerSources

When using a unified LLVM + Swift build (using `LLVM_EXTERNAL_PROJECTS=swift`),
`CMAKE_SOURCE_DIR` is `llvm-project/llvm`, and appending `include/swift` to that directory doesn’t result in the directory that contains Swift’s header files.

Instead, look up the header files relative to `CMAKE_CURRENT_SOURCE_DIR`.

roambotics/swift,roambotics/swift,apple/swift,JGiola/swift,glessard/swift,JGiola/swift,glessard/swift,benlangmuir/swift,JGiola/swift,apple/swift,glessard/swift,JGiola/swift,apple/swift,roambotics/swift,apple/swift,glessard/swift,glessard/swift,benlangmuir/swift,benlangmuir/swift,roambotics/swift,JGiola/swift,roambotics/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,JGiola/swift,roambotics/swift,apple/swift,apple/swift,benlangmuir/swift