rustc: Enable writing "unsafe extern fn() {}"

Previously, the parser would not allow you to simultaneously implement a
function with a different abi as well as being unsafe at the same time. This
extends the parser to allow functions of the form:

    unsafe extern fn foo() {
        // ...
    }

The closure type grammar was also changed to reflect this reversal, types
previously written as "extern unsafe fn()" must now be written as
"unsafe extern fn()". The parser currently has a hack which allows the old
style, but this will go away once a snapshot has landed.

Closes #10025

[breaking-change]

src/libsyntax/ast.rs

@@ -876,7 +876,6 @@ pub struct FnDecl {
 pub enum FnStyle {
     UnsafeFn, // declared with "unsafe fn"
     NormalFn, // declared with "fn"
-    ExternFn, // declared with "extern fn"
 }
 
 impl fmt::Show for FnStyle {
@@ -884,7 +883,6 @@ impl fmt::Show for FnStyle {
         match *self {
             NormalFn => "normal".fmt(f),
             UnsafeFn => "unsafe".fmt(f),
-            ExternFn => "extern".fmt(f),
         }
     }
 }