Add location to BIR::Statement of kind RETURN

Add location to BIR::Statement of kind RETURN

This commit adds location_t to BIR::Statement where type is RETURN.

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-bir-builder-expr-stmt.cc (ExprStmtBuilder::visit):
	Add location parameter.
	* checks/errors/borrowck/rust-bir-builder.h: Likewise.
	* checks/errors/borrowck/rust-bir-builder-internal.h: Add helper
	function for pushing return statements.
	* checks/errors/borrowck/rust-bir.h: Remove `expr` parameter as
	it is only needed for ASSIGNMENT statements, for which we
	already have a constructor.

Signed-off-by: Kushal Pal <kushalpal109@gmail.com>