Change prebuilt_llvm_config to not be required.

I misread this one. It is only checking if LLVM needs to be rebuilt.
There is code below that handles the case where it is unable to compute
the stamp if the source is missing.

Author: ehuss

src/bootstrap/src/core/build_steps/llvm.rs

@@ -110,7 +110,8 @@ pub fn prebuilt_llvm_config(builder: &Builder<'_>, target: TargetSelection) -> L
     }
 
     // Initialize the llvm submodule if not initialized already.
-    builder.require_and_update_submodule("src/llvm-project", None);
+    // If submodules are disabled, this does nothing.
+    builder.update_submodule("src/llvm-project");
 
     let root = "src/llvm-project/llvm";
     let out_dir = builder.llvm_out(target);