This likely affects other things too, but literals are where it appears
most. Previously, the mid-solving literal types like Array<$T0> were
just checked for whether they conformed to the protocol, without
acknowledging that this might introduce requirements on $T0, instead the
conditional requirements were checked against $T0, and so
failed (there's no reason that $T0 should satisfy the requirements at
all, it's a recently-constructed transient type variable). Instead,
capture the requirements and add them as constraints to the type
variable.
Fixes https://bugs.swift.org/browse/SR-7192 and rdar://problem/38461036
and improves https://bugs.swift.org/browse/SR-6941.
