Set link-shared if LLVM ThinLTO is enabled in config.rs

This avoids missing a shared build when uplifting LLVM artifacts into the
sysroot. We were already producing a shared link anyway, though, so this is not
a visible change from the end user's perspective.

Author: Mark-Simulacrum

src/bootstrap/compile.rs

@@ -593,7 +593,7 @@ pub fn rustc_cargo_env(builder: &Builder<'_>, cargo: &mut Cargo, target: TargetS
             let file = compiler_file(builder, builder.cxx(target).unwrap(), target, "libstdc++.a");
             cargo.env("LLVM_STATIC_STDCPP", file);
         }
-        if builder.config.llvm_link_shared || builder.config.llvm_thin_lto {
+        if builder.config.llvm_link_shared {
             cargo.env("LLVM_LINK_SHARED", "1");
         }
         if builder.config.llvm_use_libcxx {

src/bootstrap/config.rs

@@ -667,6 +667,13 @@ impl Config {
                 // CI-built LLVM is shared
                 config.llvm_link_shared = true;
             }
+
+            if config.llvm_thin_lto {
+                // If we're building with ThinLTO on, we want to link to LLVM
+                // shared, to avoid re-doing ThinLTO (which happens in the link
+                // step) with each stage.
+                config.llvm_link_shared = true;
+            }
         }
 
         if let Some(ref rust) = toml.rust {