ConstraintSystem: Remove UnviableReason::UR_MissingImport.

Now that `MemberImportVisibility` behavior is implemented using ranking there
is no need to support the `UR_MissingImport` unviability reason.

Author: tshortli

include/swift/Sema/CSFix.h

@@ -1926,14 +1926,11 @@ class MoveOutOfOrderArgument final : public ConstraintFix {
 };
 
 class AllowInaccessibleMember final : public AllowInvalidMemberRef {
-  bool IsMissingImport;
-
   AllowInaccessibleMember(ConstraintSystem &cs, Type baseType,
                           ValueDecl *member, DeclNameRef name,
-                          ConstraintLocator *locator, bool isMissingImport)
+                          ConstraintLocator *locator)
       : AllowInvalidMemberRef(cs, FixKind::AllowInaccessibleMember, baseType,
-                              member, name, locator),
-        IsMissingImport(isMissingImport) {}
+                              member, name, locator) {}
 
 public:
   std::string getName() const override {
@@ -1948,8 +1945,7 @@ class AllowInaccessibleMember final : public AllowInvalidMemberRef {
 
   static AllowInaccessibleMember *create(ConstraintSystem &cs, Type baseType,
                                          ValueDecl *member, DeclNameRef name,
-                                         ConstraintLocator *locator,
-                                         bool isMissingImport);
+                                         ConstraintLocator *locator);
 
   static bool classof(const ConstraintFix *fix) {
     return fix->getKind() == FixKind::AllowInaccessibleMember;

include/swift/Sema/ConstraintSystem.h

@@ -1980,10 +1980,6 @@ struct MemberLookupResult {
     /// The member is inaccessible (e.g. a private member in another file).
     UR_Inaccessible,
 
-    /// The member is not visible because it comes from a module that was not
-    /// imported.
-    UR_MissingImport,
-
     /// This is a `WritableKeyPath` being used to look up read-only member,
     /// used in situations involving dynamic member lookup via keypath,
     /// because it's not known upfront what access capability would the

lib/Sema/CSDiagnostics.cpp

@@ -6250,11 +6250,6 @@ bool InaccessibleMemberFailure::diagnoseAsError() {
   }
 
   auto loc = nameLoc.isValid() ? nameLoc.getStartLoc() : ::getLoc(anchor);
-  if (IsMissingImport) {
-    maybeDiagnoseMissingImportForMember(Member, getDC(), loc);
-    return true;
-  }
-
   auto accessLevel = Member->getFormalAccessScope().accessLevelForDiagnostics();
   if (auto *CD = dyn_cast<ConstructorDecl>(Member)) {
     emitDiagnosticAt(loc, diag::init_candidate_inaccessible,

lib/Sema/CSDiagnostics.h

@@ -1640,13 +1640,11 @@ class ClosureParamDestructuringFailure final : public FailureDiagnostic {
 /// ```
 class InaccessibleMemberFailure final : public FailureDiagnostic {
   ValueDecl *Member;
-  bool IsMissingImport;
 
 public:
   InaccessibleMemberFailure(const Solution &solution, ValueDecl *member,
-                            ConstraintLocator *locator, bool isMissingImport)
-      : FailureDiagnostic(solution, locator), Member(member),
-        IsMissingImport(isMissingImport) {}
+                            ConstraintLocator *locator)
+      : FailureDiagnostic(solution, locator), Member(member) {}
 
   bool diagnoseAsError() override;
 };

lib/Sema/CSFix.cpp

@@ -1153,16 +1153,16 @@ MoveOutOfOrderArgument *MoveOutOfOrderArgument::create(
 
 bool AllowInaccessibleMember::diagnose(const Solution &solution,
                                        bool asNote) const {
-  InaccessibleMemberFailure failure(solution, getMember(), getLocator(),
-                                    IsMissingImport);
+  InaccessibleMemberFailure failure(solution, getMember(), getLocator());
   return failure.diagnose(asNote);
 }
 
-AllowInaccessibleMember *AllowInaccessibleMember::create(
-    ConstraintSystem &cs, Type baseType, ValueDecl *member, DeclNameRef name,
-    ConstraintLocator *locator, bool isMissingImport) {
-  return new (cs.getAllocator()) AllowInaccessibleMember(
-      cs, baseType, member, name, locator, isMissingImport);
+AllowInaccessibleMember *
+AllowInaccessibleMember::create(ConstraintSystem &cs, Type baseType,
+                                ValueDecl *member, DeclNameRef name,
+                                ConstraintLocator *locator) {
+  return new (cs.getAllocator())
+      AllowInaccessibleMember(cs, baseType, member, name, locator);
 }
 
 bool AllowAnyObjectKeyPathRoot::diagnose(const Solution &solution,

lib/Sema/CSSimplify.cpp

@@ -10381,12 +10381,6 @@ performMemberLookup(ConstraintKind constraintKind, DeclNameRef memberName,
     if (lookupUnviable(lookupOptions | NameLookupFlags::IgnoreAccessControl,
                        MemberLookupResult::UR_Inaccessible))
       return result;
-
-    // Ignore missing import statements in order to find more candidates that
-    // might have been missed before.
-    if (lookupUnviable(lookupOptions | NameLookupFlags::IgnoreMissingImports,
-                       MemberLookupResult::UR_MissingImport))
-      return result;
   }
 
   return result;
@@ -10587,11 +10581,9 @@ static ConstraintFix *fixMemberRef(
     }
 
     case MemberLookupResult::UR_Inaccessible:
-    case MemberLookupResult::UR_MissingImport:
       assert(choice.isDecl());
-      return AllowInaccessibleMember::create(
-          cs, baseTy, choice.getDecl(), memberName, locator,
-          *reason == MemberLookupResult::UR_MissingImport);
+      return AllowInaccessibleMember::create(cs, baseTy, choice.getDecl(),
+                                             memberName, locator);
 
     case MemberLookupResult::UR_UnavailableInExistential: {
       return choice.isDecl()