Use published versions of libsyntax and librustc_errors

Rather than the versions shipping with the compiler. These are still unstable,
so nightly is still required, but we can control when we update so breakage should
happen less often.

Author: nrc

Cargo.lock

@@ -44,6 +44,8 @@ env_logger = "0.4"
 getopts = "0.2"
 derive-new = "0.5"
 cargo_metadata = "0.4"
+rustc-ap-syntax = "7.0.0"
+rustc-ap-rustc_errors = "7.0.0"
 
 [dev-dependencies]
 lazy_static = "1.0.0"