[SYCL] Fix macOS build error (#10776)

Post commit started to failed on
25c3666
(#10620) for macOS build because:

```
error: no viable overloaded '='
        ReqdWorkGroupSize = NewReqdWorkGroupSize;

note: candidate function not viable: no known conversion from 'llvm::SmallVector<size_t, 3>' (aka 'SmallVector<unsigned long, 3>') to 'const std::optional<llvm::SmallVector<unsigned long long, 3>>' for 1st argument
    _LIBCPP_INLINE_VISIBILITY optional& operator=(const optional&) = default;

Author: jzc

llvm/tools/sycl-post-link/SYCLDeviceRequirements.cpp

@@ -70,7 +70,7 @@ void llvm::getSYCLDeviceRequirements(
   std::optional<llvm::SmallVector<uint64_t, 3>> ReqdWorkGroupSize;
   for (const Function &F : MD.getModule()) {
     if (const MDNode *MDN = F.getMetadata("reqd_work_group_size")) {
-      llvm::SmallVector<size_t, 3> NewReqdWorkGroupSize;
+      llvm::SmallVector<uint64_t, 3> NewReqdWorkGroupSize;
       for (size_t I = 0, E = MDN->getNumOperands(); I < E; ++I)
         NewReqdWorkGroupSize.push_back(
             ExtractUnsignedIntegerFromMDNodeOperand(MDN, I));