Debug Info: update comments and add a FIXME.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187157 91177308-0d34-0410-b5e6-96231b3b80d8
PPC32 va_list is an actual structure so va_copy needs to copy the whole
structure not just a pointer. This implements that and thus fixes va_copy
on PPC32. Fixes #15286. Both bug and patch by Florian Zeitz!


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187158 91177308-0d34-0410-b5e6-96231b3b80d8