Only apply LTO to rustdoc at stage 2

It doesn't make much sense at stage 1, and it was broken anyway.

Author: Kobzol

Diff for: src/bootstrap/src/core/build_steps/tool.rs

@@ -660,14 +660,19 @@ impl Step for Rustdoc {
         );
 
         // rustdoc is performance sensitive, so apply LTO to it.
-        let lto = match builder.config.rust_lto {
-            RustcLto::Off => Some("off"),
-            RustcLto::Thin => Some("thin"),
-            RustcLto::Fat => Some("fat"),
-            RustcLto::ThinLocal => None,
-        };
-        if let Some(lto) = lto {
-            cargo.env(cargo_profile_var("LTO", &builder.config), lto);
+        // The compiler shared libraries (librustc_driver.so) don't use LTO on stages
+        // lower than stage 1, as it's not worth it.
+        // So we also need to only do this at stage 2+, to avoid missing LTO bitcode errors.
+        if build_compiler.stage != 0 {
+            let lto = match builder.config.rust_lto {
+                RustcLto::Off => Some("off"),
+                RustcLto::Thin => Some("thin"),
+                RustcLto::Fat => Some("fat"),
+                RustcLto::ThinLocal => None,
+            };
+            if let Some(lto) = lto {
+                cargo.env(cargo_profile_var("LTO", &builder.config), lto);
+            }
         }
 
         let _guard = builder.msg_tool(