clang doesn't emit the same warnings for global variables

[nickdesaulniers]

https://llvm.org/pr38789

[nickdesaulniers]

I plan on revisiting this later this quarter.

[nickdesaulniers]

Posting what @zygoloid sent me about this:

I think it should not require major invasive changes to Clang to fix this. While we can't built a CFG for the initializer of a global variable, we don't really need to, since there's never any control flow there (you can't have a statement-expression at global scope). So we could just walk the initializer after attaching it, skip obviously-dead branches in ?: expressions, and emit deferred runtime-behavior diagnostics for the rest. (And ideally we'd skip all of that if we don't have any pending runtime-behavior diagnostics when we reach the end of a global's initializer.)

[nickdesaulniers]

https://reviews.llvm.org/D63889