Serialize whether a VarDecl is a top-level global.

The current way that VarDecl::isLazilyInitializedGlobal() is implemented does
not work in the debugger, since the DeclContext of all VarDecls are deserialized
Swift modules. By adding a bit to the VarDecl we can recover the fact that a
VarDecl was in fact a global even in the debugger.

<rdar://problem/58939370>

Author: adrian-prantl

include/swift/AST/Decl.h

@@ -335,7 +335,7 @@ class alignas(1 << DeclAlignInBits) Decl {
     IsStatic : 1
   );
 
-  SWIFT_INLINE_BITFIELD(VarDecl, AbstractStorageDecl, 1+1+1+1+1+1+1,
+  SWIFT_INLINE_BITFIELD(VarDecl, AbstractStorageDecl, 1+1+1+1+1+1+1+1,
     /// Encodes whether this is a 'let' binding.
     Introducer : 1,
 
@@ -358,7 +358,10 @@ class alignas(1 << DeclAlignInBits) Decl {
     IsLazyStorageProperty : 1,
 
     /// Whether this is the backing storage for a property wrapper.
-    IsPropertyWrapperBackingProperty : 1
+    IsPropertyWrapperBackingProperty : 1,
+
+    /// Whether this is a lazily top-level global variable from the main file.
+    IsTopLevelGlobal : 1
   );
 
   SWIFT_INLINE_BITFIELD(ParamDecl, VarDecl, 1+2+NumDefaultArgumentKindBits,
@@ -5084,6 +5087,10 @@ class VarDecl : public AbstractStorageDecl {
     Bits.VarDecl.IsLazyStorageProperty = IsLazyStorage;
   }
 
+  /// True if this is a top-level global variable from the main source file.
+  bool isTopLevelGlobal() const { return Bits.VarDecl.IsTopLevelGlobal; }
+  void setTopLevelGlobal(bool b) { Bits.VarDecl.IsTopLevelGlobal = b; }
+  
   /// Retrieve the custom attributes that attach property wrappers to this
   /// property. The returned list contains all of the attached property wrapper attributes in source order,
   /// which means the outermost wrapper attribute is provided first.

lib/AST/Decl.cpp

@@ -5312,6 +5312,7 @@ VarDecl::VarDecl(DeclKind kind, bool isStatic, VarDecl::Introducer introducer,
   Bits.VarDecl.IsLazyStorageProperty = false;
   Bits.VarDecl.HasNonPatternBindingInit = false;
   Bits.VarDecl.IsPropertyWrapperBackingProperty = false;
+  Bits.VarDecl.IsTopLevelGlobal = false;
 }
 
 Type VarDecl::getType() const {
@@ -5410,11 +5411,7 @@ bool VarDecl::isLazilyInitializedGlobal() const {
 
   // Top-level global variables in the main source file and in the REPL are not
   // lazily initialized.
-  auto sourceFileContext = dyn_cast<SourceFile>(getDeclContext());
-  if (!sourceFileContext)
-    return true;
-
-  return !sourceFileContext->isScriptMode();
+  return !isTopLevelGlobal();
 }
 
 SourceRange VarDecl::getSourceRange() const {

lib/Parse/ParseDecl.cpp

@@ -5840,6 +5840,7 @@ Parser::parseDeclVar(ParseDeclOptions Flags,
       VD->setStatic(StaticLoc.isValid());
       VD->getAttrs() = Attributes;
       setLocalDiscriminator(VD);
+      VD->setTopLevelGlobal(topLevelDecl);
 
       // Set original declaration in `@differentiable` attributes.
       setOriginalDeclarationForDifferentiableAttributes(Attributes, VD);

lib/Serialization/Deserialization.cpp

@@ -2657,6 +2657,7 @@ class DeclDeserializer {
     uint8_t rawIntroducer;
     bool isGetterMutating, isSetterMutating;
     bool isLazyStorageProperty;
+    bool isTopLevelGlobal;
     DeclID lazyStorageID;
     unsigned numAccessors, numBackingProperties;
     uint8_t readImpl, writeImpl, readWriteImpl, opaqueReadOwnership;
@@ -2673,6 +2674,7 @@ class DeclDeserializer {
                                        hasNonPatternBindingInit,
                                        isGetterMutating, isSetterMutating,
                                        isLazyStorageProperty,
+                                       isTopLevelGlobal,
                                        lazyStorageID,
                                        opaqueReadOwnership,
                                        readImpl, writeImpl, readWriteImpl,
@@ -2804,6 +2806,7 @@ class DeclDeserializer {
     }
 
     var->setLazyStorageProperty(isLazyStorageProperty);
+    var->setTopLevelGlobal(isTopLevelGlobal);
 
     // If there are any backing properties, record them.
     if (numBackingProperties > 0) {

lib/Serialization/ModuleFormat.h

@@ -1191,6 +1191,7 @@ namespace decls_block {
     BCFixed<1>,   // is getter mutating?
     BCFixed<1>,   // is setter mutating?
     BCFixed<1>,   // is this the backing storage for a lazy property?
+    BCFixed<1>,   // top level global?
     DeclIDField,  // if this is a lazy property, this is the backing storage
     OpaqueReadOwnershipField,   // opaque read ownership
     ReadImplKindField,   // read implementation

lib/Serialization/Serialization.cpp

@@ -3255,6 +3255,7 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
                           var->isGetterMutating(),
                           var->isSetterMutating(),
                           var->isLazyStorageProperty(),
+                          var->isTopLevelGlobal(),
                           S.addDeclRef(lazyStorage),
                           accessors.OpaqueReadOwnership,
                           accessors.ReadImpl,