add support for complex tensor types in getConstantAttr

[mofeing]

diff rules that use HLOConstantFP in Enzyme-JAX are broken for complex numbers, due to mlir::enzyme::getConstantAttr not being able to handle them.

for example, the diff rule of stablehlo.rsqrt is defined as the following:

def : HLODerivative<"RsqrtOp", (Op $x), [
    (CheckedDiv (DiffeRet), (Mul (HLOConstantFP<"-2"> $x), (Mul $x, (Sqrt $x))))
]>;

which works for real numbers. but for complex numbers, like the following example,

func.func @rsqrt_complex(%x : tensor<2xcomplex<f32>>) -> tensor<2xcomplex<f32>> {
  %y = stablehlo.rsqrt %x : (tensor<2xcomplex<f32>>) -> tensor<2xcomplex<f32>>
  func.return %y : tensor<2xcomplex<f32>>
}

it errors:

mofeing@hydra:~/Enzyme-JAX$ ./bazel-bin/enzymexlamlir-opt --enzyme-wrap="infn=rsqrt_complex outfn= retTys=enzyme_dup argTys=enzyme_dup mode=ForwardMode" --enzyme-hlo-opt --cse test/lit_tests/diffrules/stablehlo/rsqrt.mlir 
 unsupported eltype: <<NULL TYPE>> of type tensor<2xcomplex<f32>>

we haven't checked this because we don't test complex numbers on diff rules that do not have a specific behaviour for them (i.e. SelectIfComplex).

this pr adds support for complex numbers in getConstantAttr. the current semantics is to create a ComplexAttr or a DenseElementsAttr with ComplexType eltype with the given value as the real part. not sure if we want to write a way to set the imaginary part.

[wsmoses]

can you add a relevant test?

[wsmoses]

you need to use mlir::Complex not std::complex