Merge #783

Merge #783

783: Get rid of lambda within `AST::TypePath` and provide a method to return a reference r=philberty a=diohabara

## Related issue

This PR will fix <https://github.com/Rust-GCC/gccrs/issues/718>

- \[x] GCC development requires copyright assignment or the Developer's Certificate of Origin sign-off, see https://gcc.gnu.org/contribute.html or https://gcc.gnu.org/dco.html
- \[x] Read contributing guidlines
- \[x] `make check-rust` passes locally
- \[x] Run `clang-format`
- \[x] Added any relevant test cases to `gcc/testsuite/rust/`

Note that you can skip the above if you are just opening a WIP PR in
order to get feedback.

Co-authored-by: Kadoi Takemaru <diohabara@gmail.com>