Add more checks for expr value in early visitors

Add more checks for expr value in early visitors

Early passes visitors may encounter constant item without a value, this
is expected as the pass rejecting a constant without an expression is
done later during the ast validation.

gcc/rust/ChangeLog:

	* expand/rust-cfg-strip.cc (CfgStrip::visit): Add expr value check.
	* expand/rust-expand-visitor.cc (ExpandVisitor::visit): Likewise.
	* resolve/rust-early-name-resolver.cc (EarlyNameResolver::visit):
	Likewise.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>