[clang] [C2y] Remove support for _Imaginary (PR #97436)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> No changelog?

I thought about adding a release note but because we never supported `_Imaginary` to begin with, it didn't seem particularly useful. Want me to add one anyway? "Clang continues to no longer support '_Imaginary', as always." ;-)

https://github.com/llvm/llvm-project/pull/97436