Silence unused variable warning in release builds

lldb/source/Core/DataFileCache.cpp:278:10: warning: unused variable 'pos' [-Wunused-variable]
    auto pos = m_string_to_offset.find(s);
         ^
lldb/source/Core/DataFileCache.cpp:277:18: warning: unused variable 'stroff' [-Wunused-variable]
    const size_t stroff = encoder.GetByteSize() - strtab_offset;
                 ^