Use variadic templates instead of relying on <cstdarg> + sentinel.

This enforces better type checking and makes code more readable.

Differential revision: https://reviews.llvm.org/D32550


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302572 91177308-0d34-0410-b5e6-96231b3b80d8