[Diagnostics] Add a distinct diagnostic for missing raw representable init

Diagnose an attempt to initialize raw representable type or convert to it
a value of some other type that matches its `RawValue` type.

```swift
enum E : Int {
   case a, b, c
}

let _: E = 0
```

`0` has to be wrapped into `E(rawValue: 0)` and either defaulted via `??` or
force unwrapped to constitute a valid binding.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -6389,3 +6389,31 @@ bool UnableToInferKeyPathRootFailure::diagnoseAsError() {
       .fixItInsertAfter(keyPathExpr->getStartLoc(), "<#Root#>");
   return true;
 }
+
+bool MissingRawRepresentativeInitFailure::diagnoseAsError() {
+  auto *locator = getLocator();
+
+  Optional<Diag<Type, Type>> message;
+
+  if (locator->isForContextualType()) {
+    message = diag::cannot_convert_initializer_value;
+  } else if (locator->isForAssignment()) {
+    message = diag::cannot_convert_assign;
+  } else if (locator->isLastElement<LocatorPathElt::ApplyArgToParam>()) {
+    message = diag::cannot_convert_argument_value;
+  }
+
+  if (!message)
+    return false;
+
+  auto diagnostic = emitDiagnostic(*message, ValueType, RawReprType);
+
+  if (auto *E = getAsExpr(getAnchor())) {
+    auto range = E->getSourceRange();
+    diagnostic
+        .fixItInsert(range.Start, RawReprType->getString() + "(rawValue: ")
+        .fixItInsertAfter(range.End, ") ?? <#default value#>");
+  }
+
+  return true;
+}

lib/Sema/CSDiagnostics.h

@@ -2102,6 +2102,33 @@ class UnableToInferKeyPathRootFailure final : public FailureDiagnostic {
   bool diagnoseAsError() override;
 };
 
+/// Diagnose an attempt to initialize raw representable type or convert to it
+/// a value of some other type that matches its `RawValue` type.
+///
+/// ```swift
+/// enum E : Int {
+///   case a, b, c
+/// }
+///
+/// let _: E = 0
+/// ```
+///
+/// `0` has to be wrapped into `E(rawValue: 0)` and either defaulted via `??` or
+/// force unwrapped to constitute a valid binding.
+class MissingRawRepresentativeInitFailure final : public FailureDiagnostic {
+  Type RawReprType;
+  Type ValueType;
+
+public:
+  MissingRawRepresentativeInitFailure(const Solution &solution,
+                                      Type rawReprType, Type valueType,
+                                      ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator), RawReprType(rawReprType),
+        ValueType(valueType) {}
+
+  bool diagnoseAsError() override;
+};
+
 } // end namespace constraints
 } // end namespace swift