[mlir] Handle the newly-added "Reserved" FramePointerKind for...

[mlir] Handle the newly-added "Reserved" FramePointerKind for 1a523925 (NFC)

/llvm-project/mlir/lib/Target/LLVMIR/ModuleImport.cpp:48:
tools/mlir/include/mlir/Dialect/LLVMIR/LLVMConversionEnumsFromLLVM.inc:158:11:
error: enumeration value 'Reserved' not handled in switch [-Werror,-Wswitch]
  switch (value) {
          ^~~~~
1 error generated.