Add a function to check if an argument list is too long.

This will be used in clang to decide if it should create an @file or not. It
will be tested on the clang side.

Patch by Nathan Froyd.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179285 91177308-0d34-0410-b5e6-96231b3b80d8