fix syntax error in suggesting generic constraint in trait parameter

suggest `where T: Foo` for the first bound on a trait, then suggest
`, T: Foo` when the suggested bound would add to an existing set of
`where` clauses. `where T: Foo` may be the first bound if `T` has a
default, because we'd rather suggest
```
trait A<T=()> where T: Copy
```
than
```
trait A<T: Copy=()>
```
for legibility reasons.

src/test/ui/trait-impl-bound-suggestions.fixed

@@ -0,0 +1,20 @@
+// run-rustfix
+
+#[allow(unused)]
+use std::fmt::Debug;
+// Rustfix should add this, or use `std::fmt::Debug` instead.
+
+#[allow(dead_code)]
+struct ConstrainedStruct<X: Copy> {
+    x: X
+}
+
+#[allow(dead_code)]
+trait InsufficientlyConstrainedGeneric<X=()> where X: Copy {
+    fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+        //~^ ERROR the trait bound `X: Copy` is not satisfied
+        ConstrainedStruct { x }
+    }
+}
+
+pub fn main() { }

src/test/ui/trait-impl-bound-suggestions.rs

@@ -0,0 +1,20 @@
+// run-rustfix
+
+#[allow(unused)]
+use std::fmt::Debug;
+// Rustfix should add this, or use `std::fmt::Debug` instead.
+
+#[allow(dead_code)]
+struct ConstrainedStruct<X: Copy> {
+    x: X
+}
+
+#[allow(dead_code)]
+trait InsufficientlyConstrainedGeneric<X=()> {
+    fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+        //~^ ERROR the trait bound `X: Copy` is not satisfied
+        ConstrainedStruct { x }
+    }
+}
+
+pub fn main() { }

src/test/ui/trait-impl-bound-suggestions.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `X: Copy` is not satisfied
+  --> $DIR/trait-impl-bound-suggestions.rs:14:52
+   |
+LL | struct ConstrainedStruct<X: Copy> {
+   |                             ---- required by this bound in `ConstrainedStruct`
+...
+LL |     fn return_the_constrained_type(&self, x: X) -> ConstrainedStruct<X> {
+   |                                                    ^^^^^^^^^^^^^^^^^^^^ the trait `Copy` is not implemented for `X`
+   |
+help: consider further restricting type parameter `X`
+   |
+LL | trait InsufficientlyConstrainedGeneric<X=()> where X: Copy {
+   |                                              ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/type/type-check-defaults.stderr

@@ -56,8 +56,8 @@ LL | trait Base<T = String>: Super<T> { }
    |
 help: consider further restricting type parameter `T`
    |
-LL | trait Base<T = String>: Super<T>, T: Copy { }
-   |                                 ^^^^^^^^^
+LL | trait Base<T = String>: Super<T> where T: Copy { }
+   |                                  ^^^^^^^^^^^^^
 
 error[E0277]: cannot add `u8` to `i32`
   --> $DIR/type-check-defaults.rs:24:66