Add language option in -gdb-show command (MI)

Summary:
Add language option in -gdb-show command + test:
```
$ bin/lldb-mi ~/p/hello
[...]
b main
[...]
r
[...]
(gdb)
-gdb-show language
^done,value="c++"
(gdb)
quit
```

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

Reviewers: abidh, granata.enrico, jingham, clayborg

Reviewed By: clayborg

Subscribers: lldb-commits, jingham, granata.enrico, clayborg, abidh

Differential Revision: http://reviews.llvm.org/D9279

llvm-svn: 235983