gccrs: Make default resolver inherit from default visitor

gccrs: Make default resolver inherit from default visitor

The default resolver put some scope in place but mostly has traversal
functions similar to the default ast visitor, making it inherit from the
default visitor allows us to avoid code duplication.

gcc/rust/ChangeLog:

	* resolve/rust-default-resolver.cc (DefaultResolver::visit): Remove
	duplicated functions.
	* resolve/rust-default-resolver.h (class DefaultResolver): Make the
	default resolver inherit from the default visitor.

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