rustdoc: Accept string source in core::run_core

This is wanted by external tooling that uses rustdoc. There are likely
some bugs when actually generating HTML output (which may expect to be
able to read the source) but all I need for now is the cleaned crate and
analysis.

src/librustdoc/lib.rs

@@ -350,8 +350,10 @@ fn rust_input(cratefile: &str, externs: core::Externs, matches: &getopts::Matche
     info!("starting to run rustc");
 
     let (mut krate, analysis) = std::thread::Thread::scoped(move |:| {
+        use rustc::session::config::Input;
+
         let cr = cr;
-        core::run_core(paths, cfgs, externs, &cr, triple)
+        core::run_core(paths, cfgs, externs, Input::File(cr), triple)
     }).join().map_err(|_| "rustc failed").unwrap();
     info!("finished with rustc");
     let mut analysis = Some(analysis);