Add a codegen option to allow loading LLVM pass plugins

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2304,6 +2304,8 @@ extern "C" {
         end_callback: SelfProfileAfterPassCallback,
         ExtraPasses: *const c_char,
         ExtraPassesLen: size_t,
+        PassPlugins: *const c_char,
+        PassPluginsLen: size_t,
     ) -> LLVMRustResult;
     pub fn LLVMRustPrintModule(
         M: &'a Module,