We were recursing infinitely in cases like this:

switch true {
case true:
	go foo.F<>
}

There were three things that came together to cause this:
1. We recently starting recursively fixing broken go/defer statements.
2. In this case we were failing to swap in the correct ast.Node in for
   the *ast.BadStmt because we were only looking
   for *ast.BlockStmt (and *ast.CaseStmt has no block).
3. After 2), we weren't returning an error so the fix() code thought
   it should recurse.

Fix 2) by using reflection to swap AST nodes in a generic way. Perhaps
a bit overkill in this case, but I happened to have already written
this for an upcoming change, so I just pulled it in to fix this bug.

Fix 3) by returning an error if we fail to swap the AST nodes.

Fixes golang/go#34353.

Change-Id: I17ff1afd52ae165c0ba9de5820dcec4cb7d756cb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/196137
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>