[build-script-impl] When skipping building LLVM in toolchain only mode, do not include FileCheck and not.

The reason why is that we will not generate targets for them causing the build
to fail.

Author: gottesmm

utils/build-script-impl

@@ -2150,8 +2150,18 @@ for host in "${ALL_HOSTS[@]}"; do
                 fi
                 if [ "${SKIP_BUILD_LLVM}" ] ; then
                     # We can't skip the build completely because the standalone
-                    # build of Swift depend on these for testing etc.
-                    build_targets=(llvm-tblgen clang-headers intrinsics_gen clang-tablegen-targets FileCheck not)
+                    # build of Swift depend on these for building and testing.
+                    build_targets=(llvm-tblgen clang-headers intrinsics_gen clang-tablegen-targets)
+                    # If we are not performing a toolchain only build, then we
+                    # also want to include FileCheck and not for testing
+                    # purposes.
+                    if [[ ! "${BUILD_TOOLCHAIN_ONLY}" ]] ; then
+                      build_targets=(
+                          "${build_targets[@]}"
+                          FileCheck
+                          not
+                      )
+                    fi
                 fi
 
                 if [ "${HOST_LIBTOOL}" ] ; then