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_metadata/messages.ftl

@@ -4,6 +4,9 @@ metadata_as_needed_compatibility =
 metadata_bad_panic_strategy =
     the linked panic runtime `{$runtime}` is not compiled with this crate's panic strategy `{$strategy}`
 
+metadata_binary_output_to_tty =
+    option `-o` or `--emit` is used to write binary output type `metadata` to stdout, but stdout is a tty
+
 metadata_bundle_needs_static =
     linking modifier `bundle` is only compatible with `static` linking kind
 
@@ -63,6 +66,9 @@ metadata_fail_seek_file =
 metadata_fail_write_file =
     failed to write to the file: {$err}
 
+metadata_failed_copy_to_stdout =
+    failed to copy {$filename} to stdout: {$err}
+
 metadata_failed_create_encoded_metadata =
     failed to create encoded metadata from file: {$err}
 
@@ -72,6 +78,9 @@ metadata_failed_create_file =
 metadata_failed_create_tempdir =
     couldn't create a temp dir: {$err}
 
+metadata_failed_remove =
+    failed to remove {$filename}: {$err}
+
 metadata_failed_write_error =
     failed to write {$filename}: {$err}