[clang] [CIR] Better handling of `void` function return (PR #128089)

[David Olsen via cfe-commits]

dkolsen-pgi wrote:

I like the idea of changing the assembly format for function types from `!cir.func<!returnType (!argType)>` to `!cir.func<(!argType) -> !returnType>`.  That is
1. Easier to parse.
2. Consistent with function types in other MLIR dialects.
3. Consistent with the assembly format for function definitions in ClangIR.

Making a change like this really needs to happen in the incubator first, not as part of upstreaming.  So my current plan is to

1. abandon this PR,
2. create a new PR in the incubator to make this change (including fixing lots of tests),
3. and then create a new upstream PR with the new behavior.


https://github.com/llvm/llvm-project/pull/128089