[ConPTY] Avoid screen clear when starting process

[Nerixyz]

In LLDB, we've been moving towards using the ConPTY to show the output of processes inline instead of a new console window:

> lldb demo.exe
...
(lldb) r
Hello World
^ would show in a new console window before
Process 1337 exited with status = 0 (0x00000000)

When starting the process and reading from the PTY, we always get the VT init-sequence that clears the screen and sets the window title (from the EchoCon example):

"\u{1b}[?9001h\u{1b}[?1004h"
"\u{1b}[?25l\u{1b}[2J\u{1b}[m\u{1b}[H\u{1b}]0;C:\\WINDOWS\\SYSTEM32\\ping.exe\u{7}\u{1b}[?25h"
"\r\nPinging
....(some output)....
Average = 0ms\r\n\u{1b}[?25h"
"\u{1b}[?9001l\u{1b}[?1004l"

• \u{1b}[?9001h: enable win32-input-mode
• \u{1b}[?1004h: send Focus In/Out events
• \u{1b}[?25l: hide cursor
• \u{1b}[2J: selective erase all
• \u{1b}[m: set default character attributes (VT100)
• \u{1b}[H: set cursor to (1,1)
• \u{1b}]0;C:\\WINDOWS\\SYSTEM32\\ping.exe\u{7}: set window title
• \u{1b}[?25h: show cursor
• output...
• \u{1b}[?9001l: disable win32-input-mode
• \u{1b}[?1004h: don't send Focus In/Out events

For LLDB, we'd like to avoid parts of the init sequence - we want to show the output of the process directly after the (lldb) r prompt. Our current solution is to filter out the sequence when starting the process.

If LLDB does not strip the init sequence, after the user runs the debuggee (r command), this is what the console window looks like:

Before running

(lldb) r

After running

Hello World
Process 1337 exited with status = 0 (0x00000000)

The (lldb) r prompt is overwritten.
Running the debuggee should not clear the screen, instead the screen should look like this:

Before running

(lldb) r

After running

(lldb) r
Hello World
Process 1337 exited with status = 0 (0x00000000)

Would it be possible to tell the pseudo-console API which parts we'd like to get or to omit some parts?