[Command] Implement `statistics` command.

[Command] Implement `statistics` command.

This allows us to collect useful metrics about lldb debugging sessions.

I thought that an example would be better than a thousand words:

  Process 19705 stopped
  * thread #1, queue = 'com.apple.main-thread', stop reason = step in
      frame #0: 0x0000000100000fb4 blah`main at blah.c:3
     1    int main(void) {
     2      int a = 6;
  -> 3      return 0;
     4    }
  (lldb) statistics enable
  (lldb) frame var a
  (int) a = 6
  (lldb) expr a
  (int) $1 = 6
  (lldb) statistics disable
  (lldb) statistics dump
  Number of expr evaluation successes : 1
  Number of expr evaluation failures : 0
  Number of frame var successes : 1
  Number of frame var failures : 0

Future improvements might include:

1. Passing a file, or implementing categories. The way this patch has
been implemented is generic enough to allow this to be extended
easily without breaking the grammar.
2. Adding an SBAPI and Python API for use in scripts.

Thanks to Jim Ingham for discussing the design with me.

<rdar://problem/36555975>

Differential Revision:  https://reviews.llvm.org/D45547

llvm-svn: 330043