incr.comp.: Store Spans as (file,line,col,length) in incr.comp. cache.

The previous method ran into problems because ICH would treat Spans
as (file,line,col) but the cache contained byte offsets and its
possible for the latter to change while the former stayed stable.

src/libsyntax_pos/lib.rs

@@ -931,6 +931,11 @@ impl FileMap {
             (lines[line_index], lines[line_index + 1])
         }
     }
+
+    #[inline]
+    pub fn contains(&self, byte_pos: BytePos) -> bool {
+        byte_pos >= self.start_pos && byte_pos <= self.end_pos
+    }
 }
 
 /// Remove utf-8 BOM if any.