[Allocator Cleanup] Move generic pointer alignment helper out of an
out-of-line private static method and into the collection of inline
alignment helpers in MathExtras.h.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204995 91177308-0d34-0410-b5e6-96231b3b80d8
[Allocator Cleanup] Sink the private data members and methods to the
bottom of the interface to make it easier to scan and find the public
API.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204996 91177308-0d34-0410-b5e6-96231b3b80d8