[TableGen] Give meaningful msg for def use in multiclass

[javed.absar]

Summary

When one mistakenly specifies 'def' instead of using 'defm', the error message presently is quite misleading: 'Couldn't find class..'

This patch makes it more clear in case the multiclass of same name does actually exist.

[javed.absar]

ping!

[hfinkel]

LGTM.

lib/TableGen/TGParser.cpp

 ///
 Record *TGParser::ParseClassID() {
   if (Lex.getCode() != tgtok::Id) {
     TokError("expected name for ClassID");
     return nullptr;
   }
 
   Record *Result = Records.getClass(Lex.getCurStrVal());
-  if (!Result)
-    TokError("Couldn't find class '" + Lex.getCurStrVal() + "'");
+  if (!Result) {
+    std::string Msg("Couldn't find class '" + Lex.getCurStrVal() + "'");
+    if (MultiClasses[Lex.getCurStrVal()].get())
+      TokError(Msg + ". Use 'defm' if you meant to use multiclass '" +
+               Lex.getCurStrVal() + "'");
+    else
+      TokError(Msg);
+  }
 
   Lex.Lex();
   return Result;
 }
 
 /// ParseMultiClassID - Parse and resolve a reference to a multiclass name.
 /// This returns null on error.
 ///

test/TableGen/MultiClass-def-fail.td

+// RUN: not llvm-tblgen %s 2>&1 | FileCheck %s
+// XFAIL: vg_leak
+
+// This test checks that using def instead of defm gives a meaningful error
+multiclass M2 {
+  def X;
+}
+
+// CHECK: error: Couldn't find class 'M2'. Use 'defm' if you meant to use multiclass 'M2'
+def rec1 : M2;