Fix use-auto-check.

[angelgarcia]

Summary

Fix a bug where use-auto check would crash when the definition of a type is in the same statement than its instantiation with new.

[alexfh]

Looks good. Thank you!

clang-tidy/modernize/UseAutoCheck.cpp

void UseAutoCheck::replaceIterators(const DeclStmt *D, ASTContext *Context) {
   // iterators but something to keep in mind in the future.
 
   SourceRange Range(V->getTypeSourceInfo()->getTypeLoc().getSourceRange());
   diag(Range.getBegin(), "use auto when declaring iterators")
       << FixItHint::CreateReplacement(Range, "auto");
 }
 
 void UseAutoCheck::replaceNew(const DeclStmt *D, ASTContext *Context) {
-  const auto *FirstDecl = cast<VarDecl>(*D->decl_begin());
+  const auto *FirstDecl = dyn_cast<VarDecl>(*D->decl_begin());
   // Ensure that there is at least one VarDecl within the DeclStmt.
   if (!FirstDecl)
     return;
 
   const QualType FirstDeclType = FirstDecl->getType().getCanonicalType();
 
   std::vector<SourceLocation> StarLocations;
   for (const auto *Dec : D->decls()) {

test/clang-tidy/modernize-use-auto-new.cpp

void auto_new() {
   MyType * const a_const = new MyType();
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: use auto when initializing with new
   // CHECK-FIXES: auto  const a_const = new MyType();
 
   MyType * volatile vol = new MyType();
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: use auto when initializing with new
   // CHECK-FIXES: auto  volatile vol = new MyType();
 
+  struct SType {} *stype = new SType;
+
   int (**func)(int, int) = new (int(*[5])(int,int));
 
   int *array = new int[5];
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: use auto when initializing with new
   // CHECK-FIXES: auto array = new int[5];
 
   MyType *ptr(new MyType);
   // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: use auto when initializing with new