Add regression test for issue 127562

The test fails in this commit. The next commit fixes it.

Author: Enselic

tests/ui/borrowck/no-invalid-mut-suggestion-for-raw-pointer-issue-127562.rs

@@ -0,0 +1,8 @@
+//! Regression test for invalid suggestion for `&raw const expr` reported in
+//! <https://github.com/rust-lang/rust/issues/127562>.
+
+fn main() {
+    let val = 2;
+    let ptr = &raw const val;
+    unsafe { *ptr = 3; } //~ ERROR cannot assign to `*ptr`, which is behind a `*const` pointer
+}

tests/ui/borrowck/no-invalid-mut-suggestion-for-raw-pointer-issue-127562.stderr

@@ -0,0 +1,14 @@
+error[E0594]: cannot assign to `*ptr`, which is behind a `*const` pointer
+  --> $DIR/no-invalid-mut-suggestion-for-raw-pointer-issue-127562.rs:7:14
+   |
+LL |     unsafe { *ptr = 3; }
+   |              ^^^^^^^^ `ptr` is a `*const` pointer, so the data it refers to cannot be written
+   |
+help: consider changing this to be a mutable pointer
+   |
+LL |     let ptr = &mut raw const val;
+   |                +++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0594`.