gccrs: Reject loop in const/static context

[Lishin1215]

Fixes #3783 .

gcc/rust/ChangeLog:

        * backend/rust-compile-expr.cc (CompileExpr::visit): add a catch for const/static

gcc/testsuite/ChangeLog:

        * rust/compile/loop_constant_context.rs: New test.
        * rust/compile/issue-3618.rs:

[Lishin1215]

In this patch I use ctx->const_context_p() to block loop in const/static,
and using !DECL_DECLARED_CONSTEXPR_P(fnctx.fndecl) to allow loop inside const fn. (like this example )

However, this may still reject the valid case mentioned here here

static _X: () = loop {
    break;
};

Thanks!

[philberty]

Good catch i raised that test case as an issue. I think lets go for your Patch and merge it for now. That test case we can handle because we could add in a check for the result of the loop if its a ! never type or () i bet we just ignore it