gccrs: add support for lang_item eq and PartialEq trait

[philberty]

The Eq and Partial Ord are very similar to the operator overloads we support for add/sub/etc... but they differ in that usually the function call name matches the name of the lang item. This time we need to have support to send in a new path for the method call on the lang item we want instead of just the name of the lang item.

NOTE: this test case doesnt work correctly yet we need to support the derive of partial eq on enums to generate the correct comparison code for that.

Fixes #3302

gcc/rust/ChangeLog:

* backend/rust-compile-expr.cc (CompileExpr::visit): handle partial_eq possible call
* backend/rust-compile-expr.h: handle case where lang item calls differ from name
* hir/tree/rust-hir-expr.cc (OperatorExprMeta::OperatorExprMeta): new helper
* hir/tree/rust-hir-expr.h: likewise
* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): handle partial_eq
(TypeCheckExpr::resolve_operator_overload): likewise
* typecheck/rust-hir-type-check-expr.h: likewise
* util/rust-lang-item.cc (LangItem::ComparisonToLangItem): map comparison to lang item
(LangItem::ComparisonToSegment): likewise
* util/rust-lang-item.h: new lang items PartialOrd and Eq
* util/rust-operators.h (enum class): likewise

gcc/testsuite/ChangeLog:

* rust/compile/cmp1.rs: New test.

[CohenArthur]

had a quick first look and it looks good, I'll review in more detail later

[philberty]

For clarity this is gimple getting generated at the end of the day:

  try          
    {
      _c1 = <test::Book as test::PartialEq::<test::BookFormat>>::eq (&b1, &D.501);  
      _c2 = <test::BookFormat as test::PartialEq::<test::Book>>::eq (&D.502, &b2); 
      _c3 = test::PartialEq::ne<Book, Book> (&b1, &b2); 
      return D.503;
    }
  finally                                               
    {                                                   
      b2 = {CLOBBER(eos)};       
      b1 = {CLOBBER(eos)};                                                                                                                                                                                                       
    }

[CohenArthur]

LGTM! thanks Philip!

[CohenArthur]

@philberty what is the part that should be derive(PartialEq) in the testcase?

[philberty]

We need it on the enum because enum comparisons are not correct because we need that derive to do it using that enum discriminant intrinsic