Add ICE regression tests

Author: estebank

src/test/ui/issues/issue-68091-unicode-ident-after-if.rs

@@ -0,0 +1,9 @@
+macro_rules! x {
+    ($($c:tt)*) => {
+        $($c)ö* {} //~ ERROR missing condition for `if` expression
+    };             //~| ERROR mismatched types
+}
+
+fn main() {
+    x!(if);
+}

src/test/ui/issues/issue-68091-unicode-ident-after-if.stderr

@@ -0,0 +1,18 @@
+error: missing condition for `if` expression
+  --> $DIR/issue-68091-unicode-ident-after-if.rs:3:14
+   |
+LL |         $($c)ö* {}
+   |              ^ expected if condition here
+
+error[E0308]: mismatched types
+  --> $DIR/issue-68091-unicode-ident-after-if.rs:3:17
+   |
+LL |         $($c)ö* {}
+   |                 ^^ expected `bool`, found `()`
+...
+LL |     x!(if);
+   |     ------- in this macro invocation
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0308`.

src/test/ui/issues/issue-68092-unicode-ident-after-incomplete-expr.rs

@@ -0,0 +1,9 @@
+macro_rules! x {
+    ($($c:tt)*) => {
+        $($c)ö* //~ ERROR macro expansion ends with an incomplete expression: expected expression
+    };
+}
+
+fn main() {
+    x!(!);
+}

src/test/ui/issues/issue-68092-unicode-ident-after-incomplete-expr.stderr

@@ -0,0 +1,8 @@
+error: macro expansion ends with an incomplete expression: expected expression
+  --> $DIR/issue-68092-unicode-ident-after-incomplete-expr.rs:3:14
+   |
+LL |         $($c)ö*
+   |              ^ expected expression
+
+error: aborting due to previous error
+