Rename the lowering dir to lower (#3172)

This is continuing with #3070, starting dir renaming with lowering
because it's at the tip. AFAICT git is catching all the file moves.

toolchain/lower/testdata/function/definition/empty_struct.carbon

@@ -0,0 +1,15 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+fn Echo(a: {}) {
+}
+
+// CHECK:STDOUT: ; ModuleID = 'empty_struct.carbon'
+// CHECK:STDOUT: source_filename = "empty_struct.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @Echo() {
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }