Revert r201734 and r201742.

This breaks backwards compatibility with existing code. Previously, this
was defined as

  #define _mm_prefetch(a, sel) (__builtin_prefetch((void *)(a), 0, (sel)))

Which basically accepts any pointer. Changing this to char* simply
breaks a lot of existing code. I have tried changing char* to
"const void*", which seems to be the right thing as per Intel
specification this should work on basically any pointer. However,
apparently this breaks windows compatibility (because of a conflicting
declaration in windows.h).

So, we probably need to #ifdef this based on whether clang is compiling
for windows. According to Chandler, this might be done by introducing an
additional symbol to a fake type in BuiltinsX86.def and then condition
the type expansion on the platform.