[clang] [clang] Move tailclipping to bitfield allocation (PR #87090)

[Nathan Sidwell via cfe-commits]

urnathan wrote:

@rjmccall can you take a look at this? as the description says, this turns out to not be NFC, but important on (probably only) LP64 machines.

https://github.com/llvm/llvm-project/pull/87090