[llvm] [llvm] Use llvm::set_is_subset (NFC) (PR #102729)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

I'm going to drop this patch.  We could use `set_is_subset` only if types are true sets without duplicates.  If `set_is_subset(A, B)` could produce a wrong result if `A` is a "set" with a lot of duplicates, making it larger than `B`.  Even if `decltype(A)` doesn't have a `size` method, `set_is_subset` could silently fail when `decltype(A)` gains a size method in the future, which is very fragile.

https://github.com/llvm/llvm-project/pull/102729