Enable LLVM assertions on more buildbots

[retep998]

A regression recently popped up which apparently would have been caught if the buildbots actually had LLVM assertions enabled rust-lang/rust#35991 (comment)

[TimNN]

cc rust-lang/rust#35998.

[alexcrichton]

@brson we already enable debug assertions on all bots, do you have any objections to just always enabling llvm assertions on all bots?

[brson]

@alexcrichton llvm assertions have a significant compile-time performance penalty. I don't personally mind taking that hit but it will slow down landing velocity.

[brson]

Also, not on the dist bots please.