gccrs: Refactor CompileDrop and add DropBuilder

[Lishin1215]

This PR refactors the Drop helper API without changing behavior.

• CompileDrop now keeps the compile Context as a member instead of passing it to each method.
• The drop candidate note/peek operations were moved from Context to DropBuilder,
  keeping the drop-specific API off the compile Context.

---

This is intended as a first step toward a dedicated Drop API.
Future work can decide whether higher-level APIs such as compile_drop_calls() should live on DropBuilder.

gcc/rust/ChangeLog:

* Make-lang.in: Add rust-compile-drop-builder.o.
* backend/rust-compile-base.cc (HIRCompileBase::compile_function_body): Update CompileDrop calls.
* backend/rust-compile-block.cc (CompileBlock::visit): Update CompileDrop calls.
* backend/rust-compile-context.h (Context): Allow DropBuilder to access drop candidate storage and move drop candidate APIs to DropBuilder.
* backend/rust-compile-drop.cc (CompileDrop::CompileDrop): Add constructor. (CompileDrop::type_has_drop_impl): Use stored Context member. (CompileDrop::compile_drop_call): Likewise. (CompileDrop::emit_current_scope_drop_calls): Use stored Context member and get drop candidates from DropBuilder.
* backend/rust-compile-drop.h: Store Context as a member.
* backend/rust-compile-pattern.cc (CompilePatternLet::visit): Use DropBuilder and update CompileDrop calls.
* backend/rust-compile-drop-builder.cc: New file.
* backend/rust-compile-drop-builder.h: New file.

[Lishin1215]

I kept block_drop_candidates owned by Context for now because it is still updated together with push_block / pop_block.

DropBuilder only wraps the note/peek operations in this PR, so the storage ownership does not change yet.

Does this split look okay as a first step?

[Lishin1215]

For this first version, I kept DropBuilder small and limited it to the drop-candidate note/peek API.

I did not move compile_drop_call into it yet, because CompileDrop still handles turning the collected candidates into actual drop calls.

Does this match the direction you had in mind for the future Drop API?

[CohenArthur]

Looks really good so far!

[CohenArthur]

Suggested change

	Context *ctx;
	Context &ctx;

[Lishin1215]

Thanks, I updated DropBuilder to store Context by reference!