Add support for 'extern const fn'

This works just as you might expect - an 'extern const fn' is a 'const
fn' that is callable from foreign code.

Currently, panicking is not allowed in consts. When RFC 2345 is
stabilized, then panicking in an 'extern const fn' will produce a
compile-time error when invoked at compile time, and an abort when
invoked at runtime.

Since this is extending the language (we're allowing the `const` keyword
in a new context), I believe that this will need an FCP. However, it's a
very minor change, so I didn't think that filing an RFC was necessary.

This will allow libc (and other FFI crates) to make many functions
`const`, without having to give up on making them `extern` as well.

src/libsyntax/parse/parser.rs

@@ -1486,6 +1486,15 @@ impl<'a> Parser<'a> {
         Ok(())
     }
 
+    /// Parses `extern` followed by an optional ABI string, or nothing.
+    fn parse_extern_abi(&mut self) -> PResult<'a, Abi> {
+        if self.eat_keyword(kw::Extern) {
+            Ok(self.parse_opt_abi()?.unwrap_or(Abi::C))
+        } else {
+            Ok(Abi::Rust)
+        }
+    }
+
     /// Parses a string as an ABI spec on an extern type or module. Consumes
     /// the `extern` keyword, if one is found.
     fn parse_opt_abi(&mut self) -> PResult<'a, Option<Abi>> {