Apply noundef metadata to loads of types that do not permit raw init

This matches the noundef attributes we apply on arguments/return types.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -442,6 +442,7 @@ pub enum MetadataType {
     MD_mem_parallel_loop_access = 10,
     MD_nonnull = 11,
     MD_type = 19,
+    MD_noundef = 29,
 }
 
 /// LLVMRustAsmDialect