[LIT] Add JSONMetricValue type to wrap types supported by the json encoder.

Summary:
The following types can be encoded and decoded by the json library:
`dict`, `list`, `tuple`, `str`, `unicode`, `int`, `long`, `float`, `bool`, `NoneType`.

`JSONMetricValue` can be constructed with any of these types, and used as part of Test.Result.
This patch also adds a toMetricValue function that converts a value into a MetricValue.




Reviewers: ddunbar, EricWF

Reviewed By: EricWF

Subscribers: cfe-commits, llvm-commits

Differential Revision: http://reviews.llvm.org/D6576