[Sema][Windows] Don't special-case void* in __unaligned conversions.

As far as I can tell, MSVC allows the relevant conversions for all
pointer types. Found compiling a Windows SDK header.

I've verified that the updated errors in MicrosoftExtensions.cpp match
the ones that MSVC actually emits, except for the one with a FIXME. (Not
sure why this wasn't done for the patch that added the tests.)

To make up for the missing error, add a warning that triggers on
conversions that drop the __unaligned qualfier.

Differential Revision: https://reviews.llvm.org/D120936