Make out arguments non-reference types.

Also, tidy some portable related code.

Change-Id: I67c8aa52eef8b556ca117ecda1b1e75465ba06a5
diff --git a/compiler/llvm/compiler_llvm.h b/compiler/llvm/compiler_llvm.h
index 65bc16b..c2211fb 100644
--- a/compiler/llvm/compiler_llvm.h
+++ b/compiler/llvm/compiler_llvm.h
@@ -70,7 +70,7 @@
     return insn_set_;
   }
 
-  void SetBitcodeFileName(std::string const& filename) {
+  void SetBitcodeFileName(const std::string& filename) {
     bitcode_filename_ = filename;
   }