Add -Zinput-stats

Emits loc, and node count - before and after expansion.

E.g.,

```
rustc: x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore
Lines of code:             32060
Pre-expansion node count:  120205
Post-expansion node count: 482749
```

src/libsyntax/codemap.rs

@@ -559,6 +559,10 @@ impl FileMap {
     pub fn is_imported(&self) -> bool {
         self.src.is_none()
     }
+
+    fn count_lines(&self) -> usize {
+        self.lines.borrow().len()
+    }
 }
 
 /// An abstraction over the fs operations used by the Parser.
@@ -1021,6 +1025,10 @@ impl CodeMap {
         debug!("span_allows_unstable? {}", allows_unstable);
         allows_unstable
     }
+
+    pub fn count_lines(&self) -> usize {
+        self.files.borrow().iter().fold(0, |a, f| a + f.count_lines())
+    }
 }
 
 // _____________________________________________________________________________