Add test

tests/codegen/dont_codegen_private_const_fn_only_used_in_const_eval.rs

@@ -0,0 +1,10 @@
+//! This test checks that we do not monomorphize functions that are only
+//! used to evaluate static items, but never used in runtime code.
+
+//@compile-flags: --crate-type=lib -Copt-level=0
+
+const fn foo() {}
+
+pub static FOO: () = foo();
+
+// CHECK: define{{.*}}foo{{.*}}