gccrs: Fix ICE with invalid const expression

gccrs: Fix ICE with invalid const expression

This patch handles the overflowed var expression in the const block, so that
we error properly in the const expr code. It was missing some stuff from the
c++ implementation in how this should be handled properly.

Fixes Rust-GCC#4139

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::compile_integer_literal): cleanup
	* backend/rust-constexpr.cc (struct constexpr_global_ctx): port over c++ helpers
	(decl_really_constant_value): likewise
	(eval_constant_expression): likewise
	(non_const_var_error): likewise

gcc/testsuite/ChangeLog:

	* rust/compile/issue-4139.rs: New test.

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