Make "Assemble stage1 compiler" orders of magnitude faster

This used to take upwards of 5 seconds for me locally. I found that the
culprit was copying the downloaded LLVM shared object:
```
[22:28:03] Install "/home/jnelson/rust-lang/rust/build/x86_64-unknown-linux-gnu/ci-llvm/lib/libLLVM-14-rust-1.62.0-nightly.so" to "/home/jnelson/rust-lang/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/lib/libLLVM-14-rust-1.62.0-nightly.so"
[22:28:09]   c Sysroot { compiler: Compiler { stage: 1, host: x86_64-unknown-linux-gnu(x86_64-unknown-linux-gnu) } }
```

It turned out that `install()` used full copies unconditionally. Change
it to use `copy()` internally, which uses hard links instead when
available.

Note that this has a change in behavior: Installing a file will also
change permissions on the source, not just the destination, if hard
links are used.

To avoid changing the behavior on symlinks for existing code, I
introduce a new function `copy_internal` which only dereferences
symlinks when told to do so.

src/bootstrap/dist.rs

@@ -1873,7 +1873,6 @@ fn add_env(builder: &Builder<'_>, cmd: &mut Command, target: TargetSelection) {
 
 /// Maybe add LLVM object files to the given destination lib-dir. Allows either static or dynamic linking.
 ///
-
 /// Returns whether the files were actually copied.
 fn maybe_install_llvm(builder: &Builder<'_>, target: TargetSelection, dst_libdir: &Path) -> bool {
     if let Some(config) = builder.config.target_config.get(&target) {
@@ -1983,6 +1982,8 @@ fn run(self, builder: &Builder<'_>) -> Option<GeneratedTarball> {
             }
         }
 
+        builder.ensure(crate::native::Llvm { target });
+
         let mut tarball = Tarball::new(builder, "llvm-tools", &target.triple);
         tarball.set_overlay(OverlayKind::LLVM);
         tarball.is_preview(true);

src/bootstrap/lib.rs

@@ -1408,6 +1408,10 @@ fn tempdir(&self) -> PathBuf {
 
     /// Copies a file from `src` to `dst`
     pub fn copy(&self, src: &Path, dst: &Path) {
+        self.copy_internal(src, dst, false);
+    }
+
+    fn copy_internal(&self, src: &Path, dst: &Path, dereference_symlinks: bool) {
         if self.config.dry_run {
             return;
         }
@@ -1417,15 +1421,22 @@ pub fn copy(&self, src: &Path, dst: &Path) {
         }
         let _ = fs::remove_file(&dst);
         let metadata = t!(src.symlink_metadata());
+        let mut src = src.to_path_buf();
         if metadata.file_type().is_symlink() {
-            let link = t!(fs::read_link(src));
-            t!(symlink_file(link, dst));
-        } else if let Ok(()) = fs::hard_link(src, dst) {
+            if dereference_symlinks {
+                src = t!(fs::canonicalize(src));
+            } else {
+                let link = t!(fs::read_link(src));
+                t!(symlink_file(link, dst));
+                return;
+            }
+        }
+        if let Ok(()) = fs::hard_link(&src, dst) {
             // Attempt to "easy copy" by creating a hard link
             // (symlinks don't work on windows), but if that fails
             // just fall back to a slow `copy` operation.
         } else {
-            if let Err(e) = fs::copy(src, dst) {
+            if let Err(e) = fs::copy(&src, dst) {
                 panic!("failed to copy `{}` to `{}`: {}", src.display(), dst.display(), e)
             }
             t!(fs::set_permissions(dst, metadata.permissions()));
@@ -1497,20 +1508,10 @@ fn install(&self, src: &Path, dstdir: &Path, perms: u32) {
         let dst = dstdir.join(src.file_name().unwrap());
         self.verbose_than(1, &format!("Install {:?} to {:?}", src, dst));
         t!(fs::create_dir_all(dstdir));
-        drop(fs::remove_file(&dst));
-        {
-            if !src.exists() {
-                panic!("Error: File \"{}\" not found!", src.display());
-            }
-            let metadata = t!(src.symlink_metadata());
-            if let Err(e) = fs::copy(&src, &dst) {
-                panic!("failed to copy `{}` to `{}`: {}", src.display(), dst.display(), e)
-            }
-            t!(fs::set_permissions(&dst, metadata.permissions()));
-            let atime = FileTime::from_last_access_time(&metadata);
-            let mtime = FileTime::from_last_modification_time(&metadata);
-            t!(filetime::set_file_times(&dst, atime, mtime));
+        if !src.exists() {
+            panic!("Error: File \"{}\" not found!", src.display());
         }
+        self.copy_internal(src, &dst, true);
         chmod(&dst, perms);
     }