InstrProf: std::to_string needs to #include <string>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228136 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/ProfileData/CoverageMappingTest.cpp

@@ -13,6 +13,8 @@
 #include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest.h"
 
+#include <string>
+
 using namespace llvm;
 using namespace coverage;