[build-script] Don't try to build llbuild Swift bindings as part of build-script.

 - llbuild builds before Swift, so we can't use the Swift compiler we are in the
   process of building for it. For now, just disable the bindings entirely.

   This will be a problem if SwiftPM ever decides that it wants to use the Swift
   bindings to manage builds more effectively.

Author: ddunbar

utils/build-script-impl

@@ -1825,6 +1825,9 @@ for deployment_target in "${HOST_TARGET}" "${CROSS_TOOLS_DEPLOYMENT_TARGETS[@]}"
                     -DFILECHECK_EXECUTABLE:PATH="$(build_directory_bin ${deployment_target} llvm)/FileCheck"
                     -DCMAKE_BUILD_TYPE:STRING="${LLBUILD_BUILD_TYPE}"
                     -DLLVM_ENABLE_ASSERTIONS:BOOL=$(true_false "${LLBUILD_ENABLE_ASSERTIONS}")
+                    # We disable all bindings, since llbuild builds before Swift
+                    # and can't use the unbuilt `swiftc`.
+                    -DLLBUILD_SUPPORT_BINDINGS:=
                 )
                 ;;
             swiftpm)