PR15662: Optimized debug info produces out of order function parameters

When a function is inlined we lazily construct the variables
representing the function's parameters. After that, we add any remaining
unused parameters.

If the function doesn't use all the parameters, or uses them out of
order, then the DWARF would produce them in that order, producing a
parameter order that doesn't match the source.

This fix causes us to always keep the arg variables at the start of the
variable list & in the original order from the source.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183297 91177308-0d34-0410-b5e6-96231b3b80d8