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/declaration/simple.carbon

@@ -0,0 +1,19 @@
+// 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 F(n: i32);
+
+fn G(n: i32) { F(n); }
+
+// CHECK:STDOUT: ; ModuleID = 'simple.carbon'
+// CHECK:STDOUT: source_filename = "simple.carbon"
+// CHECK:STDOUT:
+// CHECK:STDOUT: declare void @F(i32)
+// CHECK:STDOUT:
+// CHECK:STDOUT: define void @G(i32 %n) {
+// CHECK:STDOUT:   call void @F(i32 %n)
+// CHECK:STDOUT:   ret void
+// CHECK:STDOUT: }