Add test for unreachable well-formedness.

This commit adds a test for checking that types are well-formed when
unreachable.

src/test/ui/issue-54943.rs

@@ -0,0 +1,7 @@
+fn foo<T: 'static>() { }
+
+fn main<'a>() {
+    return;
+
+    let x = foo::<&'a u32>();
+}

src/test/ui/issue-54943.stderr

@@ -0,0 +1,11 @@
+error[E0477]: the type `&'a u32` does not fulfill the required lifetime
+  --> $DIR/issue-54943.rs:6:13
+   |
+LL |     let x = foo::<&'a u32>();
+   |             ^^^^^^^^^^^^^^
+   |
+   = note: type must satisfy the static lifetime
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0477`.