[Diagnostics] Port pattern-matching mismatch diagnostic

Port diagnostics associated with implicit use of `~=` operator
which is used in `case` statements.

lib/Sema/CSSimplify.cpp

@@ -2400,7 +2400,10 @@ bool ConstraintSystem::repairFailures(
     if (repairByInsertingExplicitCall(lhs, rhs))
       break;
 
-    if (repairViaBridgingCast(*this, lhs, rhs, conversionsOrFixes, locator))
+    bool isPatternMatching = isArgumentOfPatternMatchingOperator(loc);
+    // Let's not suggest force downcasts in pattern-matching context.
+    if (!isPatternMatching &&
+        repairViaBridgingCast(*this, lhs, rhs, conversionsOrFixes, locator))
       break;
 
     // Argument is a r-value but parameter expects an l-value e.g.
@@ -2580,6 +2583,13 @@ bool ConstraintSystem::repairFailures(
     if (hasConversionOrRestriction(ConversionRestrictionKind::DeepEquality))
       break;
 
+    // If this is a ~= operator implicitly generated by pattern matching
+    // let's not try to fix right-hand side of the operator because it's
+    // a correct contextual type.
+    if (isPatternMatching &&
+        elt.castTo<LocatorPathElt::ApplyArgToParam>().getParamIdx() == 1)
+      break;
+
     conversionsOrFixes.push_back(AllowArgumentMismatch::create(
         *this, lhs, rhs, getConstraintLocator(locator)));
     break;