[clang] [Clang] Fix BitInt padding clearing on big-endian targets (PR #215809)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This seems reasonable/about right to me, but I'd like @AaronBallman to take a look too.

https://github.com/llvm/llvm-project/pull/215809