[LLDB][Editline] empty current line before el_wgets (#165830)

This PR fixes #157637 by printing ANSI sequence "\r\x1b[K" to empty the
line before calling to `el_wgets`. Later when `el_wgets` prints the real
prompt, all wrongly printed characters are removed from the terminal.

Author: imkiva

lldb/source/Host/common/Editline.cpp

@@ -1626,6 +1626,9 @@ bool Editline::GetLine(std::string &line, bool &interrupted) {
   m_editor_status = EditorStatus::Editing;
   m_revert_cursor_index = -1;
 
+  lldbassert(m_output_stream_sp);
+  fprintf(m_locked_output->GetFile().GetStream(), "\r" ANSI_CLEAR_RIGHT);
+
   int count;
   auto input = el_wgets(m_editline, &count);
 

lldb/test/API/terminal/TestEditline.py

@@ -94,7 +94,7 @@ def test_prompt_no_color(self):
         # after the prompt.
         self.child.send("foo")
         # Check that there are no escape codes.
-        self.child.expect(re.escape("\n(lldb) foo"))
+        self.child.expect(re.escape("\n\r\x1b[K(lldb) foo"))
 
     @skipIfAsan
     @skipIfEditlineSupportMissing