gccrs: Add missing coercion site logic to return expressions

gccrs: Add missing coercion site logic to return expressions

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc: track return type in fncontext
	* backend/rust-compile-context.h (struct fncontext): likewise
	* backend/rust-compile-expr.cc (CompileExpr::visit): apply coercion site
	(CompileExpr::generate_closure_function): update push_context
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): likewise

gcc/testsuite/ChangeLog:

	* rust/execute/torture/coercion3.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>