[ClangImporter] Assert on AlternativeDecl insertion.

Adds assertion that we're not clobbering existing alternative decls,
and removes all naked accesses. NFC.

Author: milseman

lib/ClangImporter/ImportDecl.cpp

@@ -3360,13 +3360,13 @@ namespace {
           // declaration.
           if (auto imported = VisitObjCMethodDecl(decl, dc,
                                                   /*forceClassMethod=*/true))
-            Impl.AlternateDecls[result] = cast<ValueDecl>(imported);
+            Impl.setAlternateDecl(result, cast<ValueDecl>(imported));
         } else if (auto factory = importFactoryMethodAsConstructor(
                                     result, decl, selector, dc)) {
           // We imported the factory method as an initializer, so
           // record it as an alternate declaration.
           if (*factory)
-            Impl.AlternateDecls[result] = *factory;
+            Impl.setAlternateDecl(result, *factory);
         }
 
       }
@@ -4971,7 +4971,7 @@ SwiftDeclConverter::getImplicitProperty(ImportedName importedName,
                          SourceLoc());
 
   // Make the property the alternate declaration for the getter.
-  Impl.AlternateDecls[swiftGetter] = property;
+  Impl.setAlternateDecl(swiftGetter, property);
 
   return property;
 }
@@ -5021,7 +5021,7 @@ SwiftDeclConverter::importFactoryMethodAsConstructor(
   /// Record the initializer as an alternative declaration for the
   /// member.
   if (result) {
-    Impl.AlternateDecls[member] = result;
+    Impl.setAlternateDecl(member, result);
 
     if (swift3Name)
       markAsSwift2Variant(result, *swift3Name);
@@ -5736,7 +5736,7 @@ SwiftDeclConverter::importSubscript(Decl *decl,
       TypeLoc::withoutLoc(elementContextTy), dc);
 
   /// Record the subscript as an alternative declaration.
-  Impl.AlternateDecls[associateWithSetter ? setter : getter] = subscript;
+  Impl.setAlternateDecl(associateWithSetter ? setter : getter, subscript);
 
   subscript->makeComputed(SourceLoc(), getterThunk, setterThunk, nullptr,
                           SourceLoc());

lib/ClangImporter/ImporterImpl.h

@@ -427,6 +427,11 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
     }
   };
 
+  /// A mapping from imported declarations to their "alternate" declarations,
+  /// for cases where a single Clang declaration is imported to two
+  /// different Swift declarations.
+  llvm::DenseMap<Decl *, ValueDecl *> AlternateDecls;
+
 public:
   /// \brief Keep track of enum constant values that have been imported.
   llvm::DenseMap<std::pair<const clang::EnumDecl *, llvm::APSInt>,
@@ -440,11 +445,6 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
                  ConstructorDecl *>
     Constructors;
 
-  /// A mapping from imported declarations to their "alternate" declarations,
-  /// for cases where a single Clang declaration is imported to two
-  /// different Swift declarations.
-  llvm::DenseMap<Decl *, ValueDecl *> AlternateDecls;
-
   /// Retrieve the alternative declaration for the given imported
   /// Swift declaration.
   ValueDecl *getAlternateDecl(Decl *decl) {
@@ -453,6 +453,14 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
     return known->second;
   }
 
+  /// Set the alternative decl
+  void setAlternateDecl(Decl *forDecl, ValueDecl *altDecl) {
+    assert((!AlternateDecls.count(forDecl) ||
+            AlternateDecls[forDecl] == altDecl) &&
+           "clobbering already-set alternative");
+    AlternateDecls[forDecl] = altDecl;
+  }
+
 private:
   /// \brief NSObject, imported into Swift.
   Type NSObjectTy;