Made lldb.cpp build with clang 5.0.

Also removed the use of llvm::raw_string_ostream as it wasn't needed.

Also fixed a crasher that could occur when the following line returned a C string tied to a local variable:

    return OS.str().c_str();

I am guessing "static std::string buf;" was supposed to get assigned to "OS.str()" and then have "buf.c_str()" returned.

Modified the non-apple version code to cache its value and not recompute the version every time.