Add support for sub-byte aligned writes to lib/Support/Endian.h

Summary:
As per Duncan's review for D12536, I extracted the sub-byte bit aligned
reading and writing code into lib/Support, and generalized it. Added calls from
BackpatchWord. Also added unittests.

Reviewers: dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D13189

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248897 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed