[CMake] Use CMAKE_CURRENT_BINARY_DIR as base for generated C header for SwiftCompilerSources
[CMake] Use CMAKE_CURRENT_BINARY_DIR as base for generated C header for SwiftCompilerSources

When using a unified LLVM + Swift build (using `LLVM_EXTERNAL_PROJECTS=swift`), swift is installed into `build_dir/tools/swift`. Thus, we also need to find the generated headers inside the `tools/swift/include` directory and not at the top-level `build_dir/include` directory.

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