[codecompletion] Fix completion after 'let' inside a nominal type

Previously, we ignoring 'let', so you would get ridiculous completions:
  let var foo: Int
  override let func bar() {}

Now, will complete protocol requirements after 'let' the same way we do
for 'var'.  For instance property overrides, we only show them if the
'override' keyword is specified.  You can't actually override using a
'let', but if the keyword is present then the intention is clear and we
can let the user fix it afterwards when the compiler diagnoses it.

rdar://problem/31091172

lib/IDE/CodeCompletion.cpp

@@ -4096,6 +4096,11 @@ public:
       != ParsedKeywords.end();
   }
 
+  bool missingOverride(DeclVisibilityKind Reason) {
+    return !hasOverride && Reason == DeclVisibilityKind::MemberOfSuper &&
+           !CurrDeclContext->getAsProtocolOrProtocolExtensionContext();
+  }
+
   void addAccessControl(const ValueDecl *VD,
                         CodeCompletionResultBuilder &Builder) {
     assert(CurrDeclContext->getAsNominalTypeOrNominalTypeExtensionContext());
@@ -4146,10 +4151,7 @@ public:
 
     // FIXME: if we're missing 'override', but have the decl introducer we
     // should delete it and re-add both in the correct order.
-    bool missingOverride =
-      !hasOverride && Reason == DeclVisibilityKind::MemberOfSuper &&
-      !CurrDeclContext->getAsProtocolOrProtocolExtensionContext();
-    if (!hasDeclIntroducer && missingOverride)
+    if (!hasDeclIntroducer && missingOverride(Reason))
       Builder.addOverrideKeyword();
 
     if (!hasDeclIntroducer)
@@ -4168,6 +4170,14 @@ public:
   }
 
   void addVarOverride(const VarDecl *VD, DeclVisibilityKind Reason) {
+    // Overrides cannot use 'let', but if the 'override' keyword is specified
+    // then the intention is clear, so provide the results anyway.  The compiler
+    // can then provide an error telling you to use 'var' instead.
+    // If we don't need override then it's a protocol requirement, so show it.
+    if (missingOverride(Reason) && hasVarIntroducer &&
+        isKeywordSpecified("let"))
+      return;
+
     CodeCompletionResultBuilder Builder(
         Sink, CodeCompletionResult::ResultKind::Declaration,
         SemanticContextKind::Super, {});
@@ -4199,9 +4209,7 @@ public:
     if (!hasAccessModifier)
       addAccessControl(CD, Builder);
 
-    if (!hasOverride && Reason == DeclVisibilityKind::MemberOfSuper &&
-        !CurrDeclContext->getAsProtocolOrProtocolExtensionContext() &&
-        CD->isDesignatedInit() && !CD->isRequired())
+    if (missingOverride(Reason) && CD->isDesignatedInit() && !CD->isRequired())
       Builder.addOverrideKeyword();
 
     // Emit 'required' if we're in class context, 'required' is not specified,