[clang] [clang][CodeGen][UBSan] Fixing shift-exponent generation for _BitInt (PR #80515)

[Björn Pettersson via cfe-commits]

bjope wrote:

Make sure the commit message refers to fixing #80135.

(Github is a bit weird, so if you want to use the "squash an merge" support in the web UI later, then I think you need to update the first comment in the "Conversation", rather than updating the commits on this pull-request branch.)

https://github.com/llvm/llvm-project/pull/80515