[clang] [clang] Lower _BitInt(129+) to a different type in LLVM IR (PR #91364)

[John McCall via cfe-commits]

rjmccall wrote:

My experience is that compiler writers are really good at hacking in special cases to make their test cases work and really bad at recognizing that their case isn't as special as they think.  There are three types already called out for special treatment in `ConvertTypeForMem`, of which two are handled in `EmitFromMemory` and only one is handled in `EmitToMemory`.  I want to set up a pattern that the next person with this sort of problem can follow.

https://github.com/llvm/llvm-project/pull/91364