[AST] Move lazily-computed @objc bits into a separate LazySemanticInfo field.

To help us isolate the state that is primarily managed through the
request-evaluator, encapsulate that information in a LazySemanticInfo member
(as we do with superclass info and enum raw type info) rather than scattering
it around the AST.

Author: DougGregor

include/swift/AST/Decl.h

@@ -307,7 +307,7 @@ class alignas(1 << DeclAlignInBits) Decl {
     NumElements : 32
   );
 
-  SWIFT_INLINE_BITFIELD(ValueDecl, Decl, 1+1+1+1+1,
+  SWIFT_INLINE_BITFIELD(ValueDecl, Decl, 1+1+1,
     AlreadyInLookupTable : 1,
 
     /// Whether we have already checked whether this declaration is a 
@@ -316,13 +316,7 @@ class alignas(1 << DeclAlignInBits) Decl {
 
     /// Whether the decl can be accessed by swift users; for instance,
     /// a.storage for lazy var a is a decl that cannot be accessed.
-    IsUserAccessible : 1,
-
-    /// Whether the "IsObjC" bit has been computed yet.
-    IsObjCComputed : 1,
-
-    /// Whether this declaration is exposed to Objective-C.
-    IsObjC : 1
+    IsUserAccessible : 1
   );
 
   SWIFT_INLINE_BITFIELD(AbstractStorageDecl, ValueDecl, 1+1+1+1+1,
@@ -2181,6 +2175,14 @@ class ValueDecl : public Decl {
   llvm::PointerIntPair<Type, 3, OptionalEnum<AccessLevel>> TypeAndAccess;
   unsigned LocalDiscriminator = 0;
 
+  struct {
+    /// Whether the "IsObjC" bit has been computed yet.
+    unsigned isObjCComputed : 1;
+
+    /// Whether this declaration is exposed to Objective-C.
+    unsigned isObjC : 1;
+  } LazySemanticInfo;
+
 protected:
   ValueDecl(DeclKind K,
             llvm::PointerUnion<DeclContext *, ASTContext *> context,
@@ -2189,8 +2191,8 @@ class ValueDecl : public Decl {
     Bits.ValueDecl.AlreadyInLookupTable = false;
     Bits.ValueDecl.CheckedRedeclaration = false;
     Bits.ValueDecl.IsUserAccessible = true;
-    Bits.ValueDecl.IsObjCComputed = false;
-    Bits.ValueDecl.IsObjC = false;
+    LazySemanticInfo.isObjCComputed = false;
+    LazySemanticInfo.isObjC = false;
   }
 
   // MemberLookupTable borrows a bit from this type

lib/AST/Decl.cpp

@@ -1989,24 +1989,24 @@ CanType ValueDecl::getOverloadSignatureType() const {
 }
 
 bool ValueDecl::isObjC() const {
-  if (Bits.ValueDecl.IsObjCComputed)
-    return Bits.ValueDecl.IsObjC;
+  if (LazySemanticInfo.isObjCComputed)
+    return LazySemanticInfo.isObjC;
 
   // Fallback: look for an @objc attribute.
   // FIXME: This should become an error, eventually.
   return getAttrs().hasAttribute<ObjCAttr>();
 }
 
 void ValueDecl::setIsObjC(bool value) {
-  assert(!Bits.ValueDecl.IsObjCComputed || Bits.ValueDecl.IsObjC == value);
+  assert(!LazySemanticInfo.isObjCComputed || LazySemanticInfo.isObjC == value);
 
-  if (Bits.ValueDecl.IsObjCComputed) {
-    assert(Bits.ValueDecl.IsObjC == value);
+  if (LazySemanticInfo.isObjCComputed) {
+    assert(LazySemanticInfo.isObjC == value);
     return;
   }
 
-  Bits.ValueDecl.IsObjCComputed = true;
-  Bits.ValueDecl.IsObjC = value;
+  LazySemanticInfo.isObjCComputed = true;
+  LazySemanticInfo.isObjC = value;
 }
 
 bool ValueDecl::canBeAccessedByDynamicLookup() const {