Fix that AlignedAllocation.h doesn't compile because of VersionTuple

Summary:
 rL334399 put VersionTuple in the llvm namespace, but this header still assumes it's in the clang namespace.
This leads to compilation failures with enabled modules when building Clang.

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48062

llvm-svn: 334471