gccrs: Add utility function to build proc macro types
Add some utility function to build proc macro entrypoint related types.
Those functions will help generate all required metadata in order for
proc macros to be expanded properly.
gcc/rust/ChangeLog:
* backend/rust-compile.cc (build_attribute_array): Add a function to
build the attribute array type.
(build_derive_proc_macro): Add a function to build the derive proc
macro type.
(build_bang_proc_macro): Add a function to build the bang proc macro
type.
(build_attribute_proc_macro): Add a function to build the attribute
proc macro type.
(build_proc_macro): Add a function to build the proc macro tagged union
type.
(build_proc_macro_buffer): Add a function to build the proc macro
buffer type.
(build_entrypoint): Add a function to build the proc macro entrypoint
type.
* backend/rust-compile.h: Add function prototype.