[mlir][test] Fix IR/AttributeTest.cpp compilation on Solaris

The `IR/AttributeTest.cpp` test fails to compile on Solaris:

  /vol/llvm/src/llvm-project/local/mlir/unittests/IR/AttributeTest.cpp:223:36: error: no matching function for call to 'allocate'
        AttrT::get(type, "resource", UnmanagedAsmResourceBlob::allocate(data));
                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /vol/llvm/src/llvm-project/local/mlir/unittests/IR/AttributeTest.cpp:237:3: note: in instantiation of function template specialization 'checkNativeAccess<mlir::detail::DenseResourceElementsAttrBase<int8_t>, char>' requested here
    checkNativeAccess<AttrT, T>(builder.getContext(), llvm::makeArrayRef(data),
    ^
  /vol/llvm/src/llvm-project/local/mlir/unittests/IR/AttributeTest.cpp:258:3: note: in instantiation of function template specialization 'checkNativeIntAccess<mlir::detail::DenseResourceElementsAttrBase<int8_t>, char>' requested here
    checkNativeIntAccess<DenseI8ResourceElementsAttr, int8_t>(builder, 8);
    ^
  /vol/llvm/src/llvm-project/local/mlir/include/mlir/IR/AsmState.h:221:3: note: candidate template ignored: requirement '!std::is_same<char, char>::value' was not satisfied [with T = char]
    allocate(ArrayRef<T> data, bool dataIsMutable = false) {
    ^
  /vol/llvm/src/llvm-project/local/mlir/include/mlir/IR/AsmState.h:214:26: note: candidate function not viable: requires at least 2 arguments, but 1 was provided
    static AsmResourceBlob allocate(ArrayRef<char> data, size_t align,
                           ^

I suspect this happens because `char` is `signed` by default on Solaris.

Tested on `amd64-pc-solaris2.11` and `sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D131148