Debug info: Support variadic functions.
Variadic functions have an unspecified parameter tag after the last
argument. In IR this is represented as an unspecified parameter in the
subroutine type.

Paired commit with CFE.

rdar://problem/13690847

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202184 91177308-0d34-0410-b5e6-96231b3b80d8