Add missing std:: prefixes to some calls. C++ doesn't require that <cfoo>

Add missing std:: prefixes to some calls. C++ doesn't require that <cfoo>
headers provide symbols outside namespace std and the LLVM coding standards
state that we should prefix all of them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@122192 91177308-0d34-0410-b5e6-96231b3b80d8