Allow PatriciaTreeMap to be used with non-AbstractDomain values
Allow PatriciaTreeMap to be used with non-AbstractDomain values

Summary: Turns out that despite there being code comments saying that defining `Value::leq()` was optional, in practice it wouldn't compile because it was being referenced by `static_asserts` that weren't in a SFINAE context. I'm sure with enough mind-bending I could find the right incantations to make that check compile, but in practice it's easier to delete it ;)

Reviewed By: arnaudvenet

Differential Revision: D9391068

fbshipit-source-id: 02feb2c4280f02a077caed014fcb47168637bf3c

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