use the toolchain's clang when building libdispatch

Use the clang that is part of the toolchain being built
when building libdispatch to ensure getting the appropriate
version of clang.

Also pass through SWIFTC_BIN to the configure command to
prepare for a cleanup of the libdispatch side of the build process.

Author: dgrove-oss

utils/build-script-impl

@@ -2076,14 +2076,19 @@ for host in "${ALL_HOSTS[@]}"; do
             libdispatch)
                 LIBDISPATCH_BUILD_DIR=$(build_directory ${host} ${product})
                 SWIFT_BUILD_PATH="$(build_directory ${host} swift)"
+                SWIFTC_BIN="$(build_directory_bin ${LOCAL_HOST} swift)/swiftc"
+                LLVM_BIN="$(build_directory_bin ${LOCAL_HOST} llvm)"
 
                 if [[ ! -f "${LIBDISPATCH_BUILD_DIR}"/config.status ]]; then
                     # First time building; need to run autotools and configure
                     call mkdir -p "${LIBDISPATCH_BUILD_DIR}"
                     with_pushd "${LIBDISPATCH_SOURCE_DIR}" \
                         call autoreconf -fvi
                     with_pushd "${LIBDISPATCH_BUILD_DIR}" \
-                        call "${LIBDISPATCH_SOURCE_DIR}"/configure --prefix="$(get_host_install_destdir ${host})$(get_host_install_prefix ${host})" --with-swift-toolchain="${SWIFT_BUILD_PATH}"
+                               call env CC="${LLVM_BIN}/clang" CCX="${LLVM_BIN}/clang" SWIFTC="${SWIFTC_BIN}" \
+                               "${LIBDISPATCH_SOURCE_DIR}"/configure --with-swift-toolchain="${SWIFT_BUILD_PATH}" \
+                               --prefix="$(get_host_install_destdir ${host})$(get_host_install_prefix ${host})"
+
                 fi
                 with_pushd "${LIBDISPATCH_BUILD_DIR}" \
                     call make