bug: eta-expanded Prim.x calls will *not* get inlined

[crusso]

Defined as:

public let loadNat64 : (self : Region, offset : Nat64) -> Nat64 = Prim.regionLoadNat64;

loadNat64 calls will be inlined correctly:

but not when defined as:

public func loadNat64(self : Region, offset : Nat64) : Nat64 = Prim.regionLoadNat64(self, offset);

[crusso]

c.f. #5767