make borrowck more conservative around rvalues.

this will require more temporaries, but is probably less magical.
also, it means that borrowck matches trans better, so fewer crashes.
bonus.

Finally, stop warning about implicit copies when we are actually borrowing.

Also, one test (vec-res-add) stopped failing due to #2587, and hence I
added an xfail-test.

Fixes #3217, #2977, #3067

src/test/compile-fail/borrowck-loan-rcvr-overloaded-op.rs

@@ -28,11 +28,13 @@ fn b() {
 
     // Here I create an outstanding loan and check that we get conflicts:
 
-    &mut p; //~ NOTE prior loan as mutable granted here
+    let q = &mut p; //~ NOTE prior loan as mutable granted here
     //~^ NOTE prior loan as mutable granted here
 
     p + 3; //~ ERROR loan of mutable local variable as immutable conflicts with prior loan
     p.times(3); //~ ERROR loan of mutable local variable as immutable conflicts with prior loan
+
+    q.x += 1;
 }
 
 fn c() {

src/test/compile-fail/borrowck-loan-rcvr.rs

@@ -35,11 +35,13 @@ fn b() {
 
     // Here I create an outstanding loan and check that we get conflicts:
 
-    &mut p; //~ NOTE prior loan as mutable granted here
+    let l = &mut p; //~ NOTE prior loan as mutable granted here
     //~^ NOTE prior loan as mutable granted here
 
     p.purem(); //~ ERROR loan of mutable local variable as immutable conflicts with prior loan
     p.impurem(); //~ ERROR loan of mutable local variable as immutable conflicts with prior loan
+
+    l.x += 1;
 }
 
 fn c() {

src/test/compile-fail/vec-res-add.rs

@@ -1,3 +1,4 @@
+// xfail-test #2587
 // error-pattern: copying a noncopyable value
 
 struct r {