[MLIR][IRDL][irdl-to-cpp] add support for `irdl.is`

[j2kun]

In working on #158034, I came across an issue that irdl.is, which is used to constrain region block arguments, is not supported in irdl-to-cpp

error: IRDL C++ translation does not yet support translation of irdl.is operation
      %v0 = irdl.is i1
            ^
tmp/test.irdl:8:13: note: see current operation: %2 = "irdl.is"() <{expected = i1}> : () -> !irdl.attribute

[llvmbot]

@llvm/issue-subscribers-mlir

Author: Jeremy Kun (j2kun)

In working on https://github.com//issues/158034, I came across an issue that `irdl.is`, which is used to constrain region block arguments, is not supported in `irdl-to-cpp`

error: IRDL C++ translation does not yet support translation of irdl.is operation
      %v0 = irdl.is i1
            ^
tmp/test.irdl:8:13: note: see current operation: %2 = "irdl.is"() <{expected = i1}> : () -> !irdl.attribute

[Moxinilian]

I was thinking of making this part of #158040, but if this specifically can be addressed beforehand it's also fine to me.