Split off casts to void* for -Wint-to-pointer-cast to subgroup -Wint-to-void-pointer-cast.

This change is motivated from user feedback that some APIs use
void* as an opaque "context" object that may not really be a pointer.
Such users want an ability to turn off the warning for casts
to void* while preserving the warning for other cases.

Implements <rdar://problem/14016721>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182884 91177308-0d34-0410-b5e6-96231b3b80d8