Record whether a file contains a main entry point in the serialized module.

This isn't being used for much yet, but it will allow us to tell whether
something is an app target even at the module-merging stage, which is a
better way to tell if something is an app than whether it has a bridging
header.

We'll also need this if we ever take advantage of reusing "partial modules"
(serialized ASTs for other parts of the module that aren't affected by the
current source file) for compiling source files in incremental builds;
unfortunately that's unlikely given our dependency model.

Swift SVN r24531

include/swift/Serialization/ModuleFile.h

@@ -42,6 +42,8 @@ class ProtocolConformance;
 
 /// A serialized module, along with the tools to access it.
 class ModuleFile : public LazyMemberLoader {
+  friend class SerializedASTFile;
+
   /// A reference back to the AST representation of the file.
   FileUnit *FileContext = nullptr;
 
@@ -276,6 +278,13 @@ private:
   std::unique_ptr<SerializedDeclCommentTable> DeclCommentTable;
 
   struct {
+    /// The decl ID of the main class in this module file, if it has one.
+    unsigned EntryPointDeclID : 31;
+
+    /// Whether or not this module file comes from a context that had a main
+    /// entry point.
+    unsigned HasEntryPoint : 1;
+
     /// Whether this module file comes from a framework.
     unsigned IsFramework : 1;
 
@@ -288,15 +297,23 @@ private:
 
     /// Whether this module file can be used, and what's wrong if not.
     unsigned Status : 4;
+
+    // Explicitly pad out to the next word boundary.
     unsigned : 0;
   } Bits = {};
-  static_assert(sizeof(Bits) <= 4, "The bit set should be small");
+  static_assert(sizeof(Bits) <= 8, "The bit set should be small");
 
   void setStatus(ModuleStatus status) {
     Bits.Status = static_cast<unsigned>(status);
     assert(status == getStatus() && "not enough bits for status");
   }
 
+  void setEntryPointClassID(serialization::DeclID DID) {
+    Bits.HasEntryPoint = true;
+    Bits.EntryPointDeclID = DID;
+    assert(Bits.EntryPointDeclID == DID && "not enough bits for DeclID");
+  }
+
   /// Creates a new AST node to represent a deserialized decl.
   template <typename T, typename ...Args>
   T *createDecl(Args &&... args);