Replace all uses of sysroot_candidates with get_or_default_sysroot

Before this change we had two different ways to attempt to locate the
sysroot which are inconsistently used:
* get_or_default_sysroot which tries to locate based on the 0th cli
  argument and if that doesn't work falls back to locating it using the
  librustc_driver.so location and returns a single path.,
* sysroot_candidates which takes the former and additionally does
  another attempt at locating using librustc_driver.so except without
  linux multiarch handling and then returns both paths.,

The latter was originally introduced to be able to locate the codegen
backend back when cg_llvm was dynamically linked even for a custom
driver when the --sysroot passed in does not contain a copy of cg_llvm.
Back then get_or_default_sysroot did not attempt to locate the sysroot
based on the location of librustc_driver.so yet. Because that is now
done, the only case where removing sysroot_candidates can break things
is if you have a custom driver inside what looks like a sysroot
including the lib/rustlib directory, but which is missing some parts of
the full sysroot like eg rust-lld.

compiler/rustc_error_messages/src/lib.rs

@@ -107,7 +107,7 @@ impl From<Vec<FluentError>> for TranslationBundleError {
 #[instrument(level = "trace")]
 pub fn fluent_bundle(
     sysroot: PathBuf,
-    sysroot_candidates: Vec<PathBuf>,
+    default_sysroot: PathBuf,
     requested_locale: Option<LanguageIdentifier>,
     additional_ftl_path: Option<&Path>,
     with_directionality_markers: bool,
@@ -141,7 +141,7 @@ pub fn fluent_bundle(
     // If the user requests the default locale then don't try to load anything.
     if let Some(requested_locale) = requested_locale {
         let mut found_resources = false;
-        for mut sysroot in Some(sysroot).into_iter().chain(sysroot_candidates.into_iter()) {
+        for mut sysroot in [sysroot, default_sysroot] {
             sysroot.push("share");
             sysroot.push("locale");
             sysroot.push(requested_locale.to_string());