[clang] [llvm] [IR][AsmParser] Revamp how floating-point literals in LLVM IR. (PR #121838)

[Joshua Cranmer via llvm-commits]

jcranmer-intel wrote:

> As you still support the legacy format, could you please restrict this PR to only the parser changes, and leave the printer changes (and the mass test update they require) to a followup?

Sure, I can do it. I made them two separate in the commits partly for that reason.

https://github.com/llvm/llvm-project/pull/121838