[lldb][NFCI] Change parameter type in UserExpression::GetObjectPointer (#67055)

GetObjectPointer (and other related methods) do not need `ConstString`
parameters. The string parameter in these methods boil down to getting a
StringRef and calling `StackFrame::GetValueForVariableExpressionPath`
which takes a `StringRef` parameter. All the users of `GetObjectPointer`
(and related methods) end up creating ConstString objects to pass to
these methods, but they could just as easily be StringRefs (potentially
saving us some allocations in the StringPool).