cmd/compile/internal/syntax: always construct a correct syntax tree

- parser creates sensible nodes in case of syntax errors instead of nil
- a new BadExpr node is used in places where we can't do better
- fixed error message for incorrect type switch guard
- minor cleanups

Fixes #19663.

Change-Id: I028394c6db9cba7371f0e417ebf93f594659786a
Reviewed-on: https://go-review.googlesource.com/38653
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/nodes.go

@@ -125,6 +125,12 @@ type (
 		aExpr()
 	}
 
+	// Placeholder for an expression that failed to parse
+	// correctly and where we can't provide a better node.
+	BadExpr struct {
+		expr
+	}
+
 	// Value
 	Name struct {
 		Value string