Basic: correct the va_list type on Windows on ARM

Windows on ARM defines va_list as a typedef for char *.  Although the semantics
of argument passing for variadic functions matches AAPCS VFP, the wrapped
struct __va_list type is unused.  This makes the intrinsic definition for
va_list match that of Visual Studio.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212004 91177308-0d34-0410-b5e6-96231b3b80d8