SILGen: Lower formal callee type pre-opaque-type-substitution.
Prior to this fix, if we called a generic function with a substitutable opaque type as its formal substitution,
as in:
```
func opaque() -> some Any { return { } as ()->() }
func generic<T>(_ x: T) -> T { return x }
let x = generic(opaque())
```
then we would generate the formal type of the callee using the substitutions *after* opaque type expansion,
causing us to emit functions and metatypes at the wrong abstraction level and crash with SIL-level type mismatches.
Fixes rdar://problem/65683913.
