Teach TypeMember constraints to suggest force unwrapping optionals, the same way

value member constraints do.

This fixes:
<rdar://problem/21662365> QoI: diagnostic for for-each over an optional sequence isn't great

before we'd produce:
t.swift:3:10: error: '[Int]?' does not have a member named 'Generator'
for x in array {
         ^

now we produce:

t.swift:3:10: error: value of optional type '[Int]?' not unwrapped; did you mean to use '!' or '?'?
for x in array {
         ^



Swift SVN r29902

lib/Sema/CSSimplify.cpp

@@ -2981,6 +2981,25 @@ ConstraintSystem::simplifyMemberConstraint(const Constraint &constraint) {
     auto lookup = TC.lookupMemberType(DC, baseObjTy, name.getBaseName(),
                                       lookupOptions);
     if (!lookup) {
+      // If the base type was an optional, try to look through it.
+      if (shouldAttemptFixes() && baseObjTy->getOptionalObjectType()) {
+        
+        // Note the fix.
+        increaseScore(SK_Fix);
+        if (worseThanBestSolution())
+          return SolutionKind::Error;
+        
+        Fixes.push_back({FixKind::ForceOptional, constraint.getLocator()});
+        
+        // Look through one level of optional.
+        addConstraint(Constraint::create(*this, ConstraintKind::TypeMember,
+                                         baseObjTy->getOptionalObjectType(),
+                                         constraint.getSecondType(),
+                                         constraint.getMember(),
+                                         constraint.getLocator()));
+        return SolutionKind::Solved;
+      }
+      
       // FIXME: Customize diagnostic to mention types.
       recordFailure(constraint.getLocator(), Failure::DoesNotHaveMember,
                     baseObjTy, name);