Sema: correct handling for __va_start for WoA

Windows ARM indicates __va_start as a variadic function.  However, the function
itself is treated as having 4 formal arguments:
  - (out) pointer to the va_list
  - (in) address of the last named argument
  - (in) slot size for the type of the last argument
  - address of the last named argument

The last argument does not seem to have any bearing on codegen, and thus is not
explicitly type checked at this point.

Unlike the previous handling for __va_start, it does not currently validate if
the parameter is the last named parameter (it seems that MSVC currently accepts
this).

llvm-svn: 213595