gccrs: Create LocalVariable

gccrs: Create LocalVariable

This should make it easier for us to move away from leaking pointers to
Bvariable everywhere. Since LocalVariable has a single field of type
tree, it should be the same size as a pointer to Bvariable, making the
switch to LocalVariable wherever possible strictly an improvement.

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): Implicitly
	convert LocalVariable to pointer to Bvariable.
	* rust-backend.h (local_variable): Return LocalVariable.
	(parameter_variable): Likewise.
	(static_chain_variable): Likewise.
	(temporary_variable): Likewise.
	* rust-gcc.cc (local_variable): Likewise.
	(parameter_variable): Likewise.
	(static_chain_variable): Likewise.
	(temporary_variable): Likewise.
	(LocalVariable::get_tree): New function.
	(LocalVariable::error_variable): Likewise.
	* rust-gcc.h (class LocalVariable): New class.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>