[lldb] Add serial:// protocol for connecting to serial port

Add a new serial:// protocol along with SerialPort that provides a new
API to open serial ports.  The URL consists of serial device path
followed by URL-style options, e.g.:

    serial:///dev/ttyS0?baud=115200&parity=even

If no options are provided, the serial port is only set to raw mode
and the other attributes remain unchanged.  Attributes provided via
options are modified to the specified values.  Upon closing the serial
port, its original attributes are restored.

Differential Revision: https://reviews.llvm.org/D111355