[Diagnostics] Add a distinct diagnostic for use of raw representable instead of its raw value

Diagnose an attempt to pass raw representable type where its raw value
is expected instead.

```swift
enum E : Int {
  case one = 1
}

let _: Int = E.one
```

`E.one` has to use `.rawValue` to match `Int` expected by pattern binding.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -6423,3 +6423,32 @@ bool MissingRawRepresentativeInitFailure::diagnoseAsNote() {
 
   return false;
 }
+
+void UseOfRawRepresentableInsteadOfItsRawValueFailure::fixIt(
+    InFlightDiagnostic &diagnostic) const {
+  auto *E = getAsExpr(getAnchor());
+  if (!E)
+    return;
+
+  std::string fix;
+
+  auto range = E->getSourceRange();
+  if (!E->canAppendPostfixExpression()) {
+    diagnostic.fixItInsert(range.Start, "(");
+    fix += ")";
+  }
+
+  // If raw representable is an optional we need to map its raw value out
+  // out first and then, if destination is not optional, allow to specify
+  // default value.
+  if (RawReprType->getOptionalObjectType()) {
+    fix += ".map { $0.rawValue } ";
+
+    if (!ValueType->getOptionalObjectType())
+      fix += "?? <#default value#>";
+  } else {
+    fix += ".rawValue";
+  }
+
+  diagnostic.fixItInsertAfter(range.End, fix);
+}

lib/Sema/CSDiagnostics.h

@@ -2157,6 +2157,35 @@ class MissingRawRepresentativeInitFailure final
   void fixIt(InFlightDiagnostic &diagnostic) const override;
 };
 
+/// Diagnose an attempt to pass raw representable type where its raw value
+/// is expected instead.
+///
+/// ```swift
+/// enum E : Int {
+///   case one = 1
+/// }
+///
+/// let _: Int = E.one
+/// ```
+///
+/// `E.one` has to use `.rawValue` to match `Int` expected by pattern binding.
+class UseOfRawRepresentableInsteadOfItsRawValueFailure final
+    : public AbstractRawRepresentableFailure {
+public:
+  UseOfRawRepresentableInsteadOfItsRawValueFailure(const Solution &solution,
+                                                   Type rawReprType,
+                                                   Type valueType,
+                                                   ConstraintLocator *locator)
+      : AbstractRawRepresentableFailure(solution, rawReprType, valueType,
+                                        locator) {}
+
+  Type getFromType() const override { return RawReprType; }
+  Type getToType() const override { return ValueType; }
+
+private:
+  void fixIt(InFlightDiagnostic &diagnostic) const override;
+};
+
 } // end namespace constraints
 } // end namespace swift