[build-script] Pass llbuild build type arg to build-script-impl

rdar://38034197 (llbuild may not be building in release mode in the Swift toolchain)

Author: aciidgh

utils/build-script

@@ -482,6 +482,7 @@ class BuildScriptInvocation(object):
             "--libicu-build-type", args.libicu_build_variant,
             "--xctest-build-type", args.build_variant,
             "--swiftpm-build-type", args.build_variant,
+            "--llbuild-build-type", args.build_variant,
             "--swift-enable-assertions", str(args.swift_assertions).lower(),
             "--swift-stdlib-enable-assertions", str(
                 args.swift_stdlib_assertions).lower(),