simplify the logic of initializing function parameters so that we don't need
both a variable declaration and a type in FunctionArgList.  This also means
that we need to propagate the CGFunctionInfo down in a lot of places rather
than recalculating it from the FAL.  There's more we can do to eliminate
redundancy here, and I've left FIXMEs behind to do it.

llvm-svn: 127314