[Clang] [NFC] Fix potential dereferencing of nullptr (llvm#101405)

This patch replaces getAs with castAs and dyn_cast with cast to ensure
type safety and prevents potential null pointer dereferences. These
changes enforce compile-time checks for correct type casting in
ASTContext and CodeGenModule.

Author: smanna12

clang/lib/AST/ASTContext.cpp

@@ -3283,7 +3283,7 @@ static void encodeTypeForFunctionPointerAuth(const ASTContext &Ctx,
     return;
 
   case Type::Builtin: {
-    const auto *BTy = T->getAs<BuiltinType>();
+    const auto *BTy = T->castAs<BuiltinType>();
     switch (BTy->getKind()) {
 #define SIGNED_TYPE(Id, SingletonId)                                           \
   case BuiltinType::Id:                                                        \
@@ -3366,7 +3366,7 @@ static void encodeTypeForFunctionPointerAuth(const ASTContext &Ctx,
     llvm_unreachable("should never get here");
   }
   case Type::Record: {
-    const RecordDecl *RD = T->getAs<RecordType>()->getDecl();
+    const RecordDecl *RD = T->castAs<RecordType>()->getDecl();
     const IdentifierInfo *II = RD->getIdentifier();
 
     // In C++, an immediate typedef of an anonymous struct or union

clang/lib/CodeGen/CodeGenModule.cpp

@@ -5659,7 +5659,7 @@ void CodeGenModule::EmitExternalFunctionDeclaration(const FunctionDecl *FD) {
     if (getCodeGenOpts().hasReducedDebugInfo()) {
       auto *Ty = getTypes().ConvertType(FD->getType());
       StringRef MangledName = getMangledName(FD);
-      auto *Fn = dyn_cast<llvm::Function>(
+      auto *Fn = cast<llvm::Function>(
           GetOrCreateLLVMFunction(MangledName, Ty, FD, /* ForVTable */ false));
       if (!Fn->getSubprogram())
         DI->EmitFunctionDecl(FD, FD->getLocation(), FD->getType(), Fn);