gccrs: Add Drop support for block-local variable

[Lishin1215]

This PR consists of 2 commits:

For now, it handles cases like:

fn main() -> i32 {
    {
        let _x = Droppable;
    }

    0
}

---

commit 1
gccrs: add block drop candidate tracking infrastructure

gcc/rust/ChangeLog:

        * backend/rust-compile-context.h (struct DropCandidate): New struct
        for tracking block-local drop candidates.

---

commit 2
gccrs: add block-exit Drop calls

gcc/rust/ChangeLog:

        * backend/rust-compile-block.cc (compile_drop_call): New helper to
        build a drop call.
        (CompileBlock::visit): Add drop calls for candidates.
        * backend/rust-compile-pattern.cc (type_has_drop_impl): New helper to
        check whether a type implements Drop.
        (CompilePatternLet::visit): Save initialized simple drop candidates.

gcc/testsuite/ChangeLog:

        * rust/execute/drop-block-scope.rs: New test.

[Lishin1215]

For now I only handle the simple case like:

• let x = Droppable;
• let mut x = Droppable;
• let _x = Droppable;

I skipped more complex patterns for this first version, like ref and subpatterns.
I wanted to start with the smallest case first and then extend it later.
Does this scope look okay for a first draft?

[Lishin1215]

I am storing the drop candidates in Context for now.
The idea is that when we encounter an eligible variable in let, I store it in the array.
Please let me know if this is an ideal place.

[Lishin1215]

This helper finds the Drop trait, looks for the drop method, and builds the function call.
I put it in rust-compile-block.cc for now because it is only used when leaving a block.
I think this may need to move to a separate file later, since other scenarios may also need to use it.

[Lishin1215]

I added this helper to avoid getting variables whose type does not implement Drop.