AST: use a static allocated NullLocs to indicate no serialized source locations for a Decl, NFC

Author: nkcsgexi

include/swift/AST/Decl.h

@@ -672,8 +672,8 @@ class alignas(1 << DeclAlignInBits) Decl {
     SourceLoc StartLoc;
     SourceLoc EndLoc;
   };
-  mutable CachedExternalSourceLocs *CachedLocs = nullptr;
-  CachedExternalSourceLocs *calculateSerializedLocs() const;
+  mutable CachedExternalSourceLocs const *CachedLocs = nullptr;
+  const CachedExternalSourceLocs *calculateSerializedLocs() const;
 protected:
 
   Decl(DeclKind kind, llvm::PointerUnion<DeclContext *, ASTContext *> context)

lib/AST/Decl.cpp

@@ -486,14 +486,14 @@ case DeclKind::ID: return cast<ID##Decl>(this)->getLocFromSource();
   llvm_unreachable("Unknown decl kind");
 }
 
-Decl::CachedExternalSourceLocs*
-Decl::calculateSerializedLocs() const {
-  auto *Result = getASTContext().Allocate<Decl::CachedExternalSourceLocs>();
+const Decl::CachedExternalSourceLocs *Decl::calculateSerializedLocs() const {
   auto *File = cast<FileUnit>(getDeclContext()->getModuleScopeContext());
   auto Locs = File->getBasicLocsForDecl(this);
   if (!Locs.hasValue()) {
-    return Result;
+    static const Decl::CachedExternalSourceLocs NullLocs;
+    return &NullLocs;
   }
+  auto *Result = getASTContext().Allocate<Decl::CachedExternalSourceLocs>();
   auto &SM = getASTContext().SourceMgr;
 #define CASE(X)                                                               \
 Result->X = SM.getLocFromExternalSource(Locs->SourceFilePath, Locs->X.Line,   \