[basic.scope.block] Fix regression introduced by P1787R6 CWG2502

[jensmaurer]

"if (int a = 1) if (int a = 1) ..." is intended to be valid.

Fixes #4841

http://lists.isocpp.org/core/2021/08/11372.php

[tkoeppe]

Does everyone agree that this was the original intention, and that this is really editorial? @opensdh, @zygoloid?

[opensdh]

I can confirm that I didn't intend to forbid that valid C++20 code. It wasn't immediately clear to me when it was brought up on the reflector whether or not that was a "happy accident" that it prohibited bad code. Richard pointed out that use in else if is not nearly so strange, so I'm happy with Jens' fix.

[tkoeppe]

Yes, I like else if, too, one of my better-liked kinds of statement. Until recentlly I think the Google style guide had banned it (presumably inadvertently!), but I fixed that :-)