Refactor lldb-mi's prompt

Summary:
This patch fixes/cleans code around of (gdb) prompt:
# Add CMICmnStreamStdout::WritePrompt
# Clean up CMICmnStreamStdout::TextToStdout (don't lock
  the m_mutex twice because it will be locked in CMICmnStreamStdout::WritePriv)
# Remove unused CMICmnStreamStdin::m_bShowPrompt field
# Refactor CMICmnLLDBDebuggerHandleEvents to use CMICmnStreamStdout::WritePrompt
  instead of TextToStdout("(gdb)")
# Refactor CMIDriver to use CMICmnStreamStdout::WritePrompt instead of
  ```
    if (bOk && m_rStdin.GetEnablePrompt())
        bOk = m_rStdOut.WriteMIResponse(m_rStdin.GetPrompt());
  ```

Test Plan: ./dotest.py -v --executable $BUILDDIR/bin/lldb tools/lldb-mi/

Reviewers: abidh

Reviewed By: abidh

Subscribers: lldb-commits, abidh

Differential Revision: http://reviews.llvm.org/D9737

llvm-svn: 237248