Move stringpiece.h and stringprintf.h to base/.

Change-Id: I7f71b4a12f99c5f81771146c66629ae5a947b229
diff --git a/src/compiler_llvm/ir_builder.cc b/src/compiler_llvm/ir_builder.cc
index e8e73e3..51a8170 100644
--- a/src/compiler_llvm/ir_builder.cc
+++ b/src/compiler_llvm/ir_builder.cc
@@ -15,7 +15,8 @@
  */
 
 #include "ir_builder.h"
-#include "stringprintf.h"
+
+#include "base/stringprintf.h"
 
 #include <llvm/Module.h>