Refactored Builtin::Context::isPrintfLike and isScanfLike into a helper function. The implementations are identical, except for the format arguments being searched for.

No functional changes intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198446 91177308-0d34-0410-b5e6-96231b3b80d8