ICE when macro expansion has free syntactic variable reference

[pnkfelix]

% rustc --version
rustc 0.6 (77ee858 2013-02-02 11:54:29 +0100)
host: x86_64-apple-darwin

% cat bug.rs
pub fn l() {
    macro_rules! m( ($inp:expr, ) => ( $id ); );
    m!(line,)

% RUST_LOG=rustc=1,::rt::backtrace rustc bug.rs
rust: task failed at 'Key not found in table: {repr: 43}', <core-macros>:15
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug
note: try running with RUST_LOG=rustc=1,::rt::backtrace to get further 
details and report the results to github.com/mozilla/rust/issues
rust: task failed at 'explicit failure', <core-macros>:15
rust: domain main @0x7fc393818a10 root task failed

[lifthrasiir]

Same as #4040.

[graydon]

dupe of #4040