add ui test for new typeof error messages

Author: yaahc

src/test/ui/typeof/type_mismatch.rs

@@ -0,0 +1,9 @@
+// Test that using typeof results in the correct type mismatch errors instead of always assuming
+// `usize`, in addition to the pre-existing "typeof is reserved and unimplemented" error
+fn main() {
+    const a: u8 = 1;
+    let b: typeof(a) = 1i8;
+    //~^ ERROR `typeof` is a reserved keyword but unimplemented
+    //~| ERROR mismatched types
+    //~| expected `u8`, found `i8`
+}

src/test/ui/typeof/type_mismatch.stderr

@@ -0,0 +1,23 @@
+error[E0516]: `typeof` is a reserved keyword but unimplemented
+  --> $DIR/type_mismatch.rs:5:12
+   |
+LL |     let b: typeof(a) = 1i8;
+   |            ^^^^^^^^^ reserved keyword
+
+error[E0308]: mismatched types
+  --> $DIR/type_mismatch.rs:5:24
+   |
+LL |     let b: typeof(a) = 1i8;
+   |            ---------   ^^^ expected `u8`, found `i8`
+   |            |
+   |            expected due to this
+   |
+help: change the type of the numeric literal from `i8` to `u8`
+   |
+LL |     let b: typeof(a) = 1u8;
+   |                        ^^^
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0308, E0516.
+For more information about an error, try `rustc --explain E0308`.