[NFC] Fix -Wrange-loop-analysis warnings.

[NFC] Fix -Wrange-loop-analysis warnings.

Remove unnecessary `&` from loop variables.

Fix warnings: "loop variable is always a copy because the range does not
return a reference".

```
[240/2862] Building CXX object tools/mlir/tools/mlir-tblgen/CMakeFiles/mlir-tblgen.dir/TypeDefGen.cpp.o
llvm-project/mlir/tools/mlir-tblgen/TypeDefGen.cpp:50:25: warning: loop variable 'typeDef' is always a copy because the range of type 'llvm::iterator_range<llvm::mapped_iterator<std::__1::__wrap_iter<llvm::Record **>, (lambda at llvm-project/mlir/tools/mlir-tblgen/TypeDefGen.cpp:40:16), mlir::tblgen::TypeDef> >' does not return a reference [-Wrange-loop-analysis]
    for (const TypeDef &typeDef : defs)
                        ^
llvm-project/mlir/tools/mlir-tblgen/TypeDefGen.cpp:50:10: note: use non-reference type 'mlir::tblgen::TypeDef'
    for (const TypeDef &typeDef : defs)
         ^~~~~~~~~~~~~~~~~~~~~~~~
llvm-project/mlir/tools/mlir-tblgen/TypeDefGen.cpp:64:23: warning: loop variable 'typeDef' is always a copy because the range of type 'llvm::iterator_range<llvm::mapped_iterator<std::__1::__wrap_iter<llvm::Record **>, (lambda at llvm-project/mlir/tools/mlir-tblgen/TypeDefGen.cpp:40:16), mlir::tblgen::TypeDef> >' does not return a reference [-Wrange-loop-analysis]
  for (const TypeDef &typeDef : defs)
                      ^
llvm-project/mlir/tools/mlir-tblgen/TypeDefGen.cpp:64:8: note: use non-reference type 'mlir::tblgen::TypeDef'
  for (const TypeDef &typeDef : defs)
       ^~~~~~~~~~~~~~~~~~~~~~~~
2 warnings generated.

[1934/2862] Building CXX object tools...Files/toyc-ch4.dir/mlir/MLIRGen.cpp.o
llvm-project/mlir/examples/toy/Ch4/mlir/MLIRGen.cpp:139:22: warning: loop variable 'name_value' is always a copy because the range of type 'detail::zippy<detail::zip_shortest, ArrayRef<unique_ptr<VariableExprAST, default_delete<VariableExprAST> > > &, MutableArrayRef<BlockArgument> >' does not return a reference [-Wrange-loop-analysis]
    for (const auto &name_value :
                     ^
llvm-project/mlir/examples/toy/Ch4/mlir/MLIRGen.cpp:139:10: note: use non-reference type 'std::__1::tuple<const std::__1::unique_ptr<toy::VariableExprAST, std::__1::default_delete<toy::VariableExprAST> > &, mlir::BlockArgument &>'
    for (const auto &name_value :
         ^~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

[1940/2862] Building CXX object tools...Files/toyc-ch5.dir/mlir/MLIRGen.cpp.o
llvm-project/mlir/examples/toy/Ch5/mlir/MLIRGen.cpp:139:22: warning: loop variable 'name_value' is always a copy because the range of type 'detail::zippy<detail::zip_shortest, ArrayRef<unique_ptr<VariableExprAST, default_delete<VariableExprAST> > > &, MutableArrayRef<BlockArgument> >' does not return a reference [-Wrange-loop-analysis]
    for (const auto &name_value :
                     ^
llvm-project/mlir/examples/toy/Ch5/mlir/MLIRGen.cpp:139:10: note: use non-reference type 'std::__1::tuple<const std::__1::unique_ptr<toy::VariableExprAST, std::__1::default_delete<toy::VariableExprAST> > &, mlir::BlockArgument &>'
    for (const auto &name_value :
         ^~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
```

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D94003