[clang] [CIR] Upstream global initialization for ComplexType (PR #141369)

[Amr Hesham via cfe-commits]

AmrDeveloper wrote:

- This PR has implementation for the global init Complex value. I will backport it to the incubator once it is merged.

- Not totally sure if it's good idea or not to rename it to `ConstComplexAttr` similar to Const Array, Vector and Record, We can discuss it 👍🏻 

https://github.com/llvm/llvm-project/pull/141369