Give a better error message for duplicate built-in macros

Previously, this would say no such macro existed, but this was
misleading, since the macro _did_ exist, it was just already seen.

- Say where the macro was previously defined
- Add long-form error message

src/test/ui/macros/duplicate-builtin.rs

@@ -0,0 +1,17 @@
+// compile-flags:--crate-type lib
+#![feature(decl_macro)]
+#![feature(rustc_attrs)]
+
+#[rustc_builtin_macro]
+pub macro test($item:item) {
+//~^ NOTE previously defined
+    /* compiler built-in */
+}
+
+mod inner {
+    #[rustc_builtin_macro]
+    pub macro test($item:item) {
+    //~^ ERROR attempted to define built-in macro more than once [E0773]
+        /* compiler built-in */
+    }
+}

src/test/ui/macros/duplicate-builtin.stderr

@@ -0,0 +1,21 @@
+error[E0773]: attempted to define built-in macro more than once
+  --> $DIR/duplicate-builtin.rs:13:5
+   |
+LL | /     pub macro test($item:item) {
+LL | |
+LL | |         /* compiler built-in */
+LL | |     }
+   | |_____^
+   |
+note: previously defined here
+  --> $DIR/duplicate-builtin.rs:6:1
+   |
+LL | / pub macro test($item:item) {
+LL | |
+LL | |     /* compiler built-in */
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0773`.

src/test/ui/macros/unknown-builtin.rs

@@ -1,4 +1,4 @@
-// error-pattern: cannot find a built-in macro with name `line`
+// error-pattern: attempted to define built-in macro more than once
 
 #![feature(rustc_attrs)]
 
@@ -6,7 +6,7 @@
 macro_rules! unknown { () => () } //~ ERROR cannot find a built-in macro with name `unknown`
 
 #[rustc_builtin_macro]
-macro_rules! line { () => () }
+macro_rules! line { () => () } //~ NOTE previously defined here
 
 fn main() {
     line!();

src/test/ui/macros/unknown-builtin.stderr

@@ -4,7 +4,7 @@ error: cannot find a built-in macro with name `unknown`
 LL | macro_rules! unknown { () => () }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: cannot find a built-in macro with name `line`
+error[E0773]: attempted to define built-in macro more than once
   --> $SRC_DIR/core/src/macros/mod.rs:LL:COL
    |
 LL | /     macro_rules! line {
@@ -13,6 +13,13 @@ LL | |             /* compiler built-in */
 LL | |         };
 LL | |     }
    | |_____^
+   |
+note: previously defined here
+  --> $DIR/unknown-builtin.rs:9:1
+   |
+LL | macro_rules! line { () => () }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0773`.