[AST] Upgrade MainDecl to a ValueDecl

This will make it easier to store in a SILDeclRef.

Author: hamishknight

include/swift/AST/FileUnit.h

@@ -272,7 +272,7 @@ class FileUnit : public DeclContext {
     return dyn_cast_or_null<ClassDecl>(getMainDecl());
   }
   bool hasMainDecl() const { return getMainDecl(); }
-  virtual Decl *getMainDecl() const { return nullptr; }
+  virtual ValueDecl *getMainDecl() const { return nullptr; }
   FuncDecl *getMainFunc() const {
     return dyn_cast_or_null<FuncDecl>(getMainDecl());
   }

include/swift/AST/SourceFile.h

@@ -99,7 +99,7 @@ class SourceFile final : public FileUnit {
 
   /// Either the class marked \@NS/UIApplicationMain or the synthesized FuncDecl
   /// that calls main on the type marked @main.
-  Decl *MainDecl = nullptr;
+  ValueDecl *MainDecl = nullptr;
 
   /// The source location of the main type.
   SourceLoc MainDeclDiagLoc;
@@ -479,7 +479,7 @@ class SourceFile final : public FileUnit {
     llvm_unreachable("bad SourceFileKind");
   }
 
-  Decl *getMainDecl() const override { return MainDecl; }
+  ValueDecl *getMainDecl() const override { return MainDecl; }
   SourceLoc getMainDeclDiagLoc() const {
     assert(hasMainDecl());
     return MainDeclDiagLoc;
@@ -493,7 +493,7 @@ class SourceFile final : public FileUnit {
   /// one.
   ///
   /// Should only be called during type-checking.
-  bool registerMainDecl(Decl *mainDecl, SourceLoc diagLoc);
+  bool registerMainDecl(ValueDecl *mainDecl, SourceLoc diagLoc);
 
   /// True if this source file has an application entry point.
   ///

include/swift/Serialization/SerializedModuleLoader.h

@@ -432,7 +432,7 @@ class SerializedASTFile final : public LoadedFile {
 
   virtual StringRef getModuleDefiningPath() const override;
 
-  Decl *getMainDecl() const override;
+  ValueDecl *getMainDecl() const override;
 
   bool hasEntryPoint() const override;
 

lib/AST/Module.cpp

@@ -1418,7 +1418,7 @@ bool ModuleDecl::isBuiltinModule() const {
   return this == getASTContext().TheBuiltinModule;
 }
 
-bool SourceFile::registerMainDecl(Decl *mainDecl, SourceLoc diagLoc) {
+bool SourceFile::registerMainDecl(ValueDecl *mainDecl, SourceLoc diagLoc) {
   assert(mainDecl);
   if (mainDecl == MainDecl)
     return false;

lib/Serialization/ModuleFile.cpp

@@ -1237,9 +1237,9 @@ bool SerializedASTFile::getAllGenericSignatures(
   return true;
 }
 
-Decl *SerializedASTFile::getMainDecl() const {
+ValueDecl *SerializedASTFile::getMainDecl() const {
   assert(hasEntryPoint());
-  return File.getDecl(File.getEntryPointDeclID());
+  return cast_or_null<ValueDecl>(File.getDecl(File.getEntryPointDeclID()));
 }
 
 const version::Version &SerializedASTFile::getLanguageVersionBuiltWith() const {