[lldb-mi] Fix implementation for a few mi commands

Summary:
Some of the mi commands implemented in lldb-mi are incomplete/not confirming to the spec.

• gdb-show and gdb-set doesn't support getting/setting disassembly-flavor
• environment-cd should also change the working directory for inferior
• debugger CLI output should be printed as console-stream-output record, rather than being dumped directly

to stdout

• target-select should provide inner error message in mi response

Related bug report:

• https://llvm.org/bugs/show_bug.cgi?id=28026
• https://llvm.org/bugs/show_bug.cgi?id=28718
• https://llvm.org/bugs/show_bug.cgi?id=30265

Reviewers: ki.stfu, abidh

Subscribers: abidh, ki.stfu, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D24711