reorganize tests a bit

Author: RalfJung

tests/ui/consts/monomorphization/dead-code-in-called-fn.no-opt.stderr renamed to tests/ui/consts/required-consts/dead-code-in-called-fn.no-opt.stderr

@@ -13,6 +13,7 @@ impl<T> Fail<T> {
 fn called<T>() {
     // Any function that is called is guaranteed to have all consts that syntactically
     // appear in its body evaluated, even if they only appear in dead code.
+    // This relies on mono-item collection checking `required_consts` in collected functions.
     if false {
         let _ = Fail::<T>::C;
     }

tests/ui/consts/monomorphization/dead-code-in-called-fn.opt.stderr renamed to tests/ui/consts/required-consts/dead-code-in-called-fn.opt.stderr

@@ -7,7 +7,7 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered
-  --> $DIR/dead-code-in-const-called-fn.rs:16:9
+  --> $DIR/dead-code-in-const-called-fn.rs:17:9
    |
 LL |         Fail::<T>::C;
    |         ^^^^^^^^^^^^

tests/ui/consts/monomorphization/dead-code-in-called-fn.rs renamed to tests/ui/consts/required-consts/dead-code-in-called-fn.rs

@@ -7,7 +7,7 @@ LL |     const C: () = panic!();
    = note: this error originates in the macro `$crate::panic::panic_2015` which comes from the expansion of the macro `panic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 note: erroneous constant encountered
-  --> $DIR/dead-code-in-const-called-fn.rs:16:9
+  --> $DIR/dead-code-in-const-called-fn.rs:17:9
    |
 LL |         Fail::<T>::C;
    |         ^^^^^^^^^^^^

tests/ui/consts/monomorphization/dead-code-in-const-called-fn.no-opt.stderr renamed to tests/ui/consts/required-consts/dead-code-in-const-called-fn.no-opt.stderr

@@ -13,6 +13,7 @@ const fn no_codegen<T>() {
     if false {
         // This bad constant is only used in dead code in a no-codegen function... and yet we still
         // must make sure that the build fails.
+        // This relies on const-eval evaluating all `required_consts` of `const fn`.
         Fail::<T>::C; //~ constant
     }
 }