[AST] Assert that no statement/expression class is polymorphic

[riccibruno]

Summary

Add a static_assert checking that no statement/expression class is polymorphic.
People should use LLVM style RTTI instead.

[riccibruno]

Forgot a space in the error message

[aaron.ballman]

LGTM!

lib/AST/Stmt.cpp

void *Stmt::operator new(size_t bytes, const ASTContext& C,
                          unsigned alignment) {
   return ::operator new(bytes, C, alignment);
 }
 
 const char *Stmt::getStmtClassName() const {
   return getStmtInfoTableEntry((StmtClass) StmtBits.sClass).Name;
 }
 
+// Check that no statement / expression class is polymorphic. LLVM style RTTI
+// should be used instead. If absolutely needed an exception can still be added
+// here by defining the appropriate macro (but please don't do this).
+#define STMT(CLASS, PARENT) \
+  static_assert(!std::is_polymorphic<CLASS>::value, \
+                #CLASS " should not be polymorphic!");
+#include "clang/AST/StmtNodes.inc"
+
 void Stmt::PrintStats() {
   // Ensure the table is primed.
   getStmtInfoTableEntry(Stmt::NullStmtClass);
 
   unsigned sum = 0;
   llvm::errs() << "\n*** Stmt/Expr Stats:\n";
   for (int i = 0; i != Stmt::lastStmtConstant+1; i++) {
     if (StmtClassInfo[i].Name == nullptr) continue;