Module::getOrInsertFunction is using C-style vararg instead of variadic templates.

Module::getOrInsertFunction is using C-style vararg instead of variadic templates.

From a user prospective, it forces the use of an annoying nullptr to mark the end of the vararg, and there's not type checking on the arguments.
The variadic template is an obvious solution to both issues.

Differential Revision: https://reviews.llvm.org/D31070

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299949 91177308-0d34-0410-b5e6-96231b3b80d8