Add a new LLDB custom command for counting keys in debug time
Add a new LLDB custom command for counting keys in debug time

Summary:
These set of commands provide a way to use counters in debug time. By using these counters, you can track how many times the program takes a certain path.

**How to use:**
```
# Increments the counter for the key.
incrementcounter [FORMAT_STRING] [ARGS]

# Prints the counter for the key.
printcounter [FORMAT_STRING] [ARGS]

# Prints all the counters sorted by the keys.
printcounters

# Resets the counter for the key.
resetcounter [FORMAT_STRING] [ARGS]

# Resets all the counters.
resetcounters
```

**Format String:**

It uses Python's string.Formatter to format strings. You can use placeholders here as you can in Python:
https://docs.python.org/3.4/library/string.html#string.Formatter.format

**Sample Use Case:**
Let's say you have a function that logs some messages from various parts of your code.
And you want to learn how many times logMessage is called on startup.

1. Add a breakpoint to the entry point of your program (e.g. main).
   a. Add `zzz 10 printcounter` as an action.
   b. Check "Automatically continue after evaluating actions"
2. Add a breakpoint to the logMessage function.
   a. Add `incrementcounter log` as an action.
   b. Add `incrementcounter log_{} message` as an action.
   c. Check "Automatically continue after evaluating actions"
3. Run the program

Reviewed By: kolinkrewinkel

Differential Revision: D23948546

fbshipit-source-id: b2b72fe5846ba890ea37d0d9a252279833a69ba2

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