[mlir][NFC] Move CodeGenHelpers.cpp from mlir-tblgen to TableGen library

Its header was already part of the TableGen library, but unusable as uses of its functions or classes would lead to undefined references when linking. This fixes that.

mlir/lib/TableGen/CMakeLists.txt

@@ -14,6 +14,7 @@ llvm_add_library(MLIRTableGen STATIC
   AttrOrTypeDef.cpp
   Builder.cpp
   Class.cpp
+  CodeGenHelpers.cpp
   Constraint.cpp
   Dialect.cpp
   Format.cpp

mlir/tools/mlir-tblgen/CMakeLists.txt

@@ -9,7 +9,6 @@ add_tablegen(mlir-tblgen MLIR
   EXPORT MLIR
   AttrOrTypeDefGen.cpp
   AttrOrTypeFormatGen.cpp
-  CodeGenHelpers.cpp
   DialectGen.cpp
   DirectiveCommonGen.cpp
   EnumsGen.cpp