Rather than using std::pair, create a structure to represent the type. This is
a preliminary refactoring to enable required parameter handling. Additional
state is needed to indicate required parameters. This has a minor side effect
of improving readability by providing more accurate names compared to first and
second.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201629 91177308-0d34-0410-b5e6-96231b3b80d8