Add ui/borrowck/borrowck-closures-mut-of-mut.rs.

This is a variant of `ui/borrowck/borrowck-closures-mut-of-imm.rs`
that I used to help identify what changes I needed to make to the
latter file in order to recover its instances of E0524 under NLL.

(Basically this test includes the changes you'd need to make to
`ui/borrowck/borrowck-closures-mut-of-imm.rs` in order to get rid of
occurrences of E0596. And then I realized that one needs to add
invocations of the closures in order to properly extend the mutable
reborrows in a manner such that NLL will roughly match AST-borrowck.)

src/test/ui/borrowck/borrowck-closures-mut-of-mut.nll.stderr

@@ -0,0 +1,18 @@
+error[E0524]: two closures require unique access to `x` at the same time
+  --> $DIR/borrowck-closures-mut-of-mut.rs:14:18
+   |
+LL |     let mut c1 = || set(&mut *x);
+   |                  --           - first borrow occurs due to use of `x` in closure
+   |                  |
+   |                  first closure is constructed here
+LL |     let mut c2 = || set(&mut *x);
+   |                  ^^           - second borrow occurs due to use of `x` in closure
+   |                  |
+   |                  second closure is constructed here
+LL |     //~^ ERROR two closures require unique access to `x` at the same time
+LL |     c2(); c1();
+   |           -- first borrow later used here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0524`.

src/test/ui/borrowck/borrowck-closures-mut-of-mut.rs

@@ -0,0 +1,20 @@
+// Tests that two closures cannot simultaneously both have mutable
+// access to the variable. Related to issue #6801.
+
+fn get(x: &isize) -> isize {
+    *x
+}
+
+fn set(x: &mut isize) {
+    *x = 4;
+}
+
+fn a(x: &mut isize) {
+    let mut c1 = || set(&mut *x);
+    let mut c2 = || set(&mut *x);
+    //~^ ERROR two closures require unique access to `x` at the same time
+    c2(); c1();
+}
+
+fn main() {
+}

src/test/ui/borrowck/borrowck-closures-mut-of-mut.stderr

@@ -0,0 +1,18 @@
+error[E0524]: two closures require unique access to `x` at the same time
+  --> $DIR/borrowck-closures-mut-of-mut.rs:14:18
+   |
+LL |     let mut c1 = || set(&mut *x);
+   |                  --           - previous borrow occurs due to use of `x` in closure
+   |                  |
+   |                  first closure is constructed here
+LL |     let mut c2 = || set(&mut *x);
+   |                  ^^           - borrow occurs due to use of `x` in closure
+   |                  |
+   |                  second closure is constructed here
+...
+LL | }
+   | - borrow from first closure ends here
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0524`.