driver: Use atty instead of rolling our own

Rationale:

- `atty` is widely used in the Rust ecosystem
- We already use it (in `rustc_errors` and other places)
- We shouldn't be rolling our own TTY detector when there's a
  widely-used, well-tested package that we can use

Cargo.lock

@@ -3652,6 +3652,7 @@ dependencies = [
 name = "rustc_driver"
 version = "0.0.0"
 dependencies = [
+ "atty",
  "libc",
  "rustc_ast",
  "rustc_ast_pretty",

compiler/rustc_driver/Cargo.toml

@@ -9,6 +9,7 @@ crate-type = ["dylib"]
 
 [dependencies]
 libc = "0.2"
+atty = "0.2"
 tracing = { version = "0.1.18" }
 tracing-subscriber = { version = "0.2.13", default-features = false, features = ["fmt", "env-filter", "smallvec", "parking_lot", "ansi"] }
 tracing-tree = "0.1.6"

compiler/rustc_driver/src/lib.rs

@@ -546,43 +546,12 @@ impl Compilation {
 #[derive(Copy, Clone)]
 pub struct RustcDefaultCalls;
 
-// FIXME remove these and use winapi 0.3 instead
-// Duplicates: bootstrap/compile.rs, librustc_errors/emitter.rs
-#[cfg(unix)]
 fn stdout_isatty() -> bool {
-    unsafe { libc::isatty(libc::STDOUT_FILENO) != 0 }
+    atty::is(atty::Stream::Stdout)
 }
 
-#[cfg(windows)]
-fn stdout_isatty() -> bool {
-    use winapi::um::consoleapi::GetConsoleMode;
-    use winapi::um::processenv::GetStdHandle;
-    use winapi::um::winbase::STD_OUTPUT_HANDLE;
-
-    unsafe {
-        let handle = GetStdHandle(STD_OUTPUT_HANDLE);
-        let mut out = 0;
-        GetConsoleMode(handle, &mut out) != 0
-    }
-}
-
-// FIXME remove these and use winapi 0.3 instead
-#[cfg(unix)]
 fn stderr_isatty() -> bool {
-    unsafe { libc::isatty(libc::STDERR_FILENO) != 0 }
-}
-
-#[cfg(windows)]
-fn stderr_isatty() -> bool {
-    use winapi::um::consoleapi::GetConsoleMode;
-    use winapi::um::processenv::GetStdHandle;
-    use winapi::um::winbase::STD_ERROR_HANDLE;
-
-    unsafe {
-        let handle = GetStdHandle(STD_ERROR_HANDLE);
-        let mut out = 0;
-        GetConsoleMode(handle, &mut out) != 0
-    }
+    atty::is(atty::Stream::Stderr)
 }
 
 fn handle_explain(registry: Registry, code: &str, output: ErrorOutputType) {