Auto merge of #111626 - pjhades:output, r=b-naber

Write to stdout if `-` is given as output file

With this PR, if `-o -` or `--emit KIND=-` is provided, output will be written to stdout instead. Binary output (those of type `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.

This implements rust-lang/compiler-team#431

The idea behind the changes is to introduce an `OutFileName` enum that represents the output - be it a real path or stdout - and to use this enum along the code paths that handle different output types.

Author: bors