gccrs: add checks for division by zero and left shift overflow

gccrs: add checks for division by zero and left shift overflow

These are ported from the c-family code c-warn.cc and c/c-typchk.cc

Fixes Rust-GCC#2394

gcc/rust/ChangeLog:

	* backend/rust-constexpr.cc (eval_store_expression): check for null
	(eval_call_expression): remove bad warning
	* rust-gcc.cc (arithmetic_or_logical_expression): add warnings

gcc/testsuite/ChangeLog:

	* rust/compile/issue-2394.rs: New test.

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