Write to stdout if - is given as output file

If `-o -` or `--emit KIND=-` is provided, output will be written
to stdout instead. Binary output (`obj`, `llvm-bc`, `link` and
`metadata`) being written this way will result in an error unless
stdout is not a tty. Multiple output types going to stdout will
trigger an error too, as they will all be mixded together.

compiler/rustc_interface/src/errors.rs

@@ -108,3 +108,7 @@ pub struct IgnoringExtraFilename;
 #[derive(Diagnostic)]
 #[diag(interface_ignoring_out_dir)]
 pub struct IgnoringOutDir;
+
+#[derive(Diagnostic)]
+#[diag(interface_multiple_output_types_to_stdout)]
+pub struct MultipleOutputTypesToStdout;