[AST] [analyzer] NFC: Reuse code in stable ID dumping methods.

Use the new fancy method introduced in r348197 to simplify some code.

Differential Revision: https://reviews.llvm.org/D54488

llvm-svn: 348199
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index f65d306..95babf7 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -955,10 +955,7 @@
 static Decl::Kind getKind(const DeclContext *DC) { return DC->getDeclKind(); }
 
 int64_t Decl::getID() const {
-  Optional<int64_t> Out = getASTContext().getAllocator().identifyObject(this);
-  assert(Out && "Wrong allocator used");
-  assert(*Out % alignof(Decl) == 0 && "Wrong alignment information");
-  return *Out / alignof(Decl);
+  return getASTContext().getAllocator().identifyKnownAlignedObject<Decl>(this);
 }
 
 const FunctionType *Decl::getFunctionType(bool BlocksToo) const {