IR/UseTest.cpp: Avoid std::to_string() to appease mingw32 bot.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213994 91177308-0d34-0410-b5e6-96231b3b80d8
Update X86/Utils/LLVMBuild.txt corresponding to r213986. "Core" has been introduced.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213995 91177308-0d34-0410-b5e6-96231b3b80d8