compiler: suppress type descriptor generation for aliases

compiler: suppress type descriptor generation for aliases

    Change Named_object::get_backend to ignore aliases when creating type
    descriptors for types, to be consistent with
    Type::needs_specific_type_functions and the Specific_type_functions
    traversal class. For example, when compiling a package that creates an
    alias to an an externally defined type, e.g.

       import "foo"
       type MyFoo = foo.Foo

    it makes sense to skip the alias (not try to generate type specific
    functions for it) and let the normal mechanisms take care of the alias
    target, depending on whether the target is defined locally or defined
    elsewhere.

    Testcase for this problen can be found in CL 193261.

    Fixes golang/go#33866.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/191961

From-SVN: r275651