[clang] [NFC][analyzer] Eliminate SwitchNodeBuilder (PR #188096)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

I wanted to do a few cosmetic changes before merging this PR, but as I was tweaking the comment before the `!DefaultBlock)` branch, I spotted a logic error which I fixed in https://github.com/llvm/llvm-project/pull/188096/commits/ffbb916d02357084bb4104d0c9c0c1d72623df8a.

It seems that I need to be more careful when I'm inlining functions... The buggy logic was introduced in https://github.com/llvm/llvm-project/pull/188096/commits/6d9c0cb612008dfd0a179e53f7adf36cbb1c850c when I turned `return nullptr;` into `return;` instead of `continue;`.

https://github.com/llvm/llvm-project/pull/188096