swift234

[ClangImporter] Replace one hack to deal with va_list for another.

va_list on i386 and arm64 is a pointer. On armv7, it's a struct that
/contains/ a pointer. But on x86_64, it's an array of one struct element,
which means it decays to a pointer as an argument but not as a variable or
struct field.

This patch:
- Looks for a decayed pointer argument that represents an x86_64 va_list.
- Validates that the va_list on other platforms is at least pointer-sized.
- Uses the decayed (pointer) type when getting the Clang type for
  CVaListPointer.
- Drops the implicit conversion from CVaListPointer to COpaquePointer.

All together, this provides more type-safety for functions using va_list on
x86_64.

Swift SVN r17307