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

[Adam Magier via cfe-commits]

AdamMagierFOSS wrote:

One thing I'll preemptively address is I didn't know where to put the new unit testing - creating a separate file seems a little heavy handed but I see that there's a test for UBSan shift generation (`clang/test/CodeGen/ubsan-shift.c`) and one for UBSan + _BitInt (`clang/test/CodeGen/ext-int-sanitizer.cpp`). Both seem equally "valid" but neither seem to test in the same way that I'm trying to test these changes. Advice on this would be appreciated.

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