[lldb] Handle SIGINT via the MainLoop signal thread (on POSIX) (#195959)

The driver's async SIGINT handler called
SBDebugger::DispatchInputInterrupt directly. That is not
async-signal-safe and can lead to a crash.

Register SIGINT with the existing signal-thread MainLoop instead so
DispatchInputInterrupt runs in normal thread context. The Windows path
is unchanged and keeps the legacy async handler.

While DispatchInputInterrupt runs, the callback temporarily installs
SIG_DFL so a second Ctrl-C still hard-terminates the process, preserving
the escape hatch users rely on when the debugger is unresponsive.

Moving SIGINT off the main thread means a Ctrl-C no longer interrupts
blocking syscalls there (e.g. a Python REPL waiting on input or
sleeping), so Python never observes the queued interrupt and
KeyboardInterrupt is not raised. To restore that behavior, after
dispatching the interrupt the callback re-raises SIGINT on the main
thread via pthread_kill; the resulting EINTR lets Python pick up the
pending interrupt. A skip flag suppresses the re-entry that this
self-send produces. Because the callback only ever runs on the signal
thread, the flag and the captured main-thread id live in the lambda's
captures and need no synchronization.

rdar://158218595