[llvm] fix some warnings in SmallPtrSetTest (PR #77956)

[Isaac Turner via llvm-commits]

spacey-sooty wrote:

> > Warnings are related to places which should use braces
> 
> Could you paste a few of these warnings to here? I don't see warnings with my build and I'm curious what the compiler is complaining about.
> 
> In general, the llvm coding style doesn't require braces around single-line if/for statements like this: https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements.
> 
> I think it makes sense to add these with `for (...) if (...) ... ; else (...);` like you did in a few places, but not to add it everywhere.

I probably should've specified the warnings were from Clangd. The warnings were a lot of "Statement should be inside braces (fix available) [readability-braces-around-statement]"

https://github.com/llvm/llvm-project/pull/77956