Modify "statistics dump" to dump JSON.

Modify "statistics dump" to dump JSON.

This patch is a smaller version of a previous patch https://reviews.llvm.org/D110804.

This patch modifies the output of "statistics dump" to be able to get stats from the current target. It adds 3 new stats as well. The output of "statistics dump" is now emitted as JSON so that it can be used to track performance and statistics and the output could be used to populate a database that tracks performance. Sample output looks like:

(lldb) statistics dump
{
  "expressionEvaluation": {
    "failures": 0,
    "successes": 0
  },
  "firstStopTime": 0.34164492800000001,
  "frameVariable": {
    "failures": 0,
    "successes": 0
  },
  "launchOrAttachTime": 0.31969605400000001,
  "targetCreateTime": 0.0040863039999999998
}

The top level keys are:

"expressionEvaluation" which replaces the previous stats that were emitted as plain text. This dictionary contains the success and fail counts.
"frameVariable" which replaces the previous stats for "frame variable" that were emitted as plain text. This dictionary contains the success and fail counts.
"targetCreateTime" contains the number of seconds it took to create the target and load dependent libraries (if they were enabled) and also will contain symbol preloading times if that setting is enabled.
"launchOrAttachTime" is the time it takes from when the launch/attach is initiated to when the first private stop occurs.
"firstStopTime" is the time in seconds that it takes to stop at the first stop that is presented to the user via the LLDB interface. This value will only have meaning if you set a known breakpoint or stop location in your code that you want to measure as a performance test.

This diff is also meant as a place to discuess what we want out of the "statistics dump" command before adding more funcionality. It is also meant to clean up the previous code that was storting statistics in a vector of numbers within the lldb_private::Target class.

Differential Revision: https://reviews.llvm.org/D111686