[lldb] Encode `bool` as unsigned int

`bool` is considered to be unsigned according to `std::is_unsigned<bool>::value` (and `Type::GetTypeInfo`). Encoding it as signed int works fine for normal variables and fields, but breaks when reading the values of boolean bitfields. If the field is declared as `bool b : 1` and has a value of `0b1`, the call to `SBValue::GetValueAsSigned()` will return `-1`.

Reviewed By: teemperor

Differential Revision: https://reviews.llvm.org/D102685