[Diagnostics] Support diagnosing references to operators without argument application

[hborla]

in MissingConformanceFailure::diagnoseAsAmbiguousOperatorRef

When there's an ambiguous operator reference that manifests as a missing conformance failure, such as:

func myAssertion<T>(_ a: T, _ op: ((T,T)->Bool), _ b: T) {}
var e: Error? = nil
myAssertion(e, ==, nil)

This previously would crash because the code assumed the parent of the operator ref was an ApplyExpr.

This will also print a note pointing to the candidate with the conformance requirement.

Resolves: rdar://problem/60727310

[hborla]

@swift-ci please smoke test