Add --dep-info to write Makefile-compatible dependency info.

When --dep-info is given, rustc will write out a `$input_base.d` file in the
output directory that contains Makefile compatible dependency information for
use with tools like make and ninja.

src/libsyntax/codemap.rs

@@ -260,6 +260,10 @@ impl FileMap {
         };
         self.multibyte_chars.push(mbc);
     }
+
+    pub fn is_real_file(&self) -> bool {
+        !(self.name.starts_with("<") && self.name.ends_with(">"))
+    }
 }
 
 pub struct CodeMap {