Point to type argument span when used as trait

Given the following code:

```rust
struct Foo<T: Clone>(T);

use std::ops::Add;

impl<T: Clone, Add> Add for Foo<T> {
    type Output = usize;

    fn add(self, rhs: Self) -> Self::Output {
      unimplemented!();
    }
}
```

present the following output:

```nocode
error[E0404]: `Add` is not a trait
 --> file3.rs:5:21
  |
5 | impl<T: Clone, Add> Add for Okok<T> {
  |                ---  ^^^ expected trait, found type parameter
  |                |
  |                type parameter defined here
```

1 files changed, 8 insertions, 0 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 30fc4c3dd80..cc39e81fdae 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -399,6 +399,14 @@ impl Generics {
     pub fn is_parameterized(&self) -> bool {
         self.is_lt_parameterized() || self.is_type_parameterized()
     }
+    pub fn span_for_name(&self, name: &str) -> Option<Span> {
+        for t in &self.ty_params {
+            if t.ident.name.as_str() == name {
+                return Some(t.span);
+            }
+        }
+        None
+    }
 }
 
 impl Default for Generics {