Disallow dereferencing of void* in C++.

as Discussed:
https://discourse.llvm.org/t/rfc-can-we-stop-the-extension-to-allow-dereferencing-void-in-c/65708

There is no good reason to allow this when the other compilers all
reject this, and it messes with SFINAE/constraint checking.

Differential Revision: https://reviews.llvm.org/D135287