gccrs: Change cfg stripper to use default visitor

gccrs: Change cfg stripper to use default visitor

Cfg visitor used it's own visit patterns, this made the code longer than
required.

gcc/rust/ChangeLog:

	* expand/rust-cfg-strip.cc (CfgStrip::visit): Change calls from visitor
	to default visitor.
	(CfgStrip::go): Add call to visit crate.
	* expand/rust-cfg-strip.h (class CfgStrip): Update prototypes and
	remove empty ones.
	* ast/rust-ast-visitor.cc: add WhereClause condition check.

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