Parser: fix PatIdent span bug

Fix small bug introduced in e38cb972dcf: PatIdent span was incorrect
because self.last_span was being used before the ident token was
parsed.
auto merge of #15428 : phildawes/rust/master, r=huonw

Fix small bug introduced in e38cb972dcf: PatIdent span was incorrect
because self.last_span was being used before the ident token was
parsed.