[MLIR] Fix LLVMIRTransforms build failure

lib/libMLIRLLVMIRTransforms.a fails to build from scratch with the
following error:
In file included from llvm/include/llvm/Frontend/OpenMP/OMPConstants.h:19,
                 from llvm/include/llvm/Frontend/OpenMP/OMPIRBuilder.h:19,
                 from mlir/include/mlir/Target/LLVMIR/ModuleTranslation.h:26,
                 from mlir/include/mlir/Dialect/LLVMIR/NVVMDialect.h:24,
                 from mlir/lib/Dialect/LLVMIR/Transforms/InlinerInterfaceImpl.cpp:17:
llvm/include/llvm/Frontend/OpenMP/OMP.h:16:10:
fatal error: llvm/Frontend/OpenMP/OMP.h.inc: No such file or directory

Use a forward declaration for OpenMPIRBuilder in ModuleTranslation.h to
avoid pulling OpenMP frontend header that require generated headers.

Author: RoboTux

mlir/include/mlir/Target/LLVMIR/ModuleTranslation.h

@@ -23,12 +23,13 @@
 #include "mlir/Target/LLVMIR/TypeToLLVM.h"
 
 #include "llvm/ADT/SetVector.h"
-#include "llvm/Frontend/OpenMP/OMPIRBuilder.h"
+#include "llvm/IR/FPEnv.h"
 
 namespace llvm {
 class BasicBlock;
-class IRBuilderBase;
 class Function;
+class IRBuilderBase;
+class OpenMPIRBuilder;
 class Value;
 } // namespace llvm