[flang][debug] Support complex types. (#92559)

This PR adds supports for conversion of complex type to corresponding
DITypeAttr. Both fir and mlir types are supported.

Apart from lit testing, I have also tested the types in debugger and
they work correctly. An exception is 128 bit complex which somehow
requires that its name be different from `complex`. I am going to open a
separate PR to add (kind=n) in the type names similar to what gfortran
does.