Support x test --stage 1 ui-fulldeps

Nils had an excellent idea the other day: the same way that rustdoc is
able to load `rustc_driver` from the sysroot, ui-fulldeps tests should
also be able to load it from the sysroot. That allows us to run fulldeps
tests with stage1, without having to fully rebuild the compiler twice.
It does unfortunately have the downside that we're running the tests on
the *bootstrap* compiler, not the in-tree sources, but since most of the
fulldeps tests are for the *API* of the compiler, that seems ok.

I think it's possible to extend this to `run-make-fulldeps`, but I've
run out of energy for tonight.

- Move `plugin` tests into a subdirectory.

  Plugins are loaded at runtime with `dlopen` and so require the ABI of
  the running compile to match the ABI of the compiler linked with
  `rustc_driver`. As a result they can't be supported in stage 1 and have
  to use `// ignore-stage1`.

- Remove `ignore-stage1` from most non-plugin tests

- Ignore diagnostic tests in stage 1. Even though this requires a stage
  2 build to load rustc_driver, it's primarily testing the error message
  that the *running* compiler emits when the diagnostic struct is malformed.

- Pass `-Zforce-unstable-if-unmarked` in stage1, not just stage2. That
  allows running `hash-stable-is-unstable` in stage1, since it now
  suggests adding `rustc_private` to enable loading the crates.

- Add libLLVM.so to the stage0 target sysroot, to allow fulldeps tests
  that act as custom drivers to load it at runtime.

- Pass `--sysroot stage0-sysroot` in compiletest so that we use the
  correct version of std.

Author: jyn514

src/bootstrap/bin/rustc.rs

@@ -150,7 +150,7 @@ fn main() {
     // allow the `rustc_private` feature to link to other unstable crates
     // also in the sysroot. We also do this for host crates, since those
     // may be proc macros, in which case we might ship them.
-    if env::var_os("RUSTC_FORCE_UNSTABLE").is_some() && (stage != "0" || target.is_some()) {
+    if env::var_os("RUSTC_FORCE_UNSTABLE").is_some() {
         cmd.arg("-Z").arg("force-unstable-if-unmarked");
     }
 

src/bootstrap/builder.rs

@@ -1008,9 +1008,24 @@ impl<'a> Builder<'a> {
                 // Avoid deleting the rustlib/ directory we just copied
                 // (in `impl Step for Sysroot`).
                 if !builder.download_rustc() {
+                    builder.verbose(&format!(
+                        "Removing sysroot {} to avoid caching bugs",
+                        sysroot.display()
+                    ));
                     let _ = fs::remove_dir_all(&sysroot);
                     t!(fs::create_dir_all(&sysroot));
                 }
+
+                if self.compiler.stage == 0 {
+                    // The stage 0 compiler for the build triple is always pre-built.
+                    // Ensure that `libLLVM.so` ends up in the target libdir, so that ui-fulldeps tests can use it when run.
+                    dist::maybe_install_llvm_target(
+                        builder,
+                        self.compiler.host,
+                        &builder.sysroot(self.compiler),
+                    );
+                }
+
                 INTERNER.intern_path(sysroot)
             }
         }