Detect and diagnose a contextual mismatch between expected
collection element type and the one provided (e.g. source
of the assignment or argument to a call) e.g.:
```swift
let _: [Int] = ["hello"]
func foo(_: [Int]) {}
foo(["hello"])
```
Detect that failed requirement comes from contextual type and use
that information to determine affected declaration.
Resolves: rdar://problem/47980354
Extend existing `RequirementFailure` functionality to support
conditional requirement failures. Such fixes are introduced
only if the parent type requirement has been matched successfully.
Resolves: rdar://problem/47871590
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.
