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 r202185.

rdar://problem/13690847

This re-applies r202184 + a bugfix in DwarfDebug's argument handling.

llvm-svn: 202188