[lit] Use sharding for GoogleTest format

This helps lit unit test performance by a lot, especially on windows. The performance gain comes from launching one gtest executable for many subtests instead of one (this is the current situation).

The shards are executed by the test runner and the results are stored in the
json format supported by the GoogleTest. Later in the test reporting stage,
all test results in the json file are retrieved to continue the test results
summary etc.

On my Win10 desktop, before this patch: `check-clang-unit`: 177s, `check-llvm-unit`: 38s; after this patch: `check-clang-unit`: 37s, `check-llvm-unit`: 11s.
On my Linux machine, before this patch: `check-clang-unit`: 46s, `check-llvm-unit`: 8s; after this patch: `check-clang-unit`: 7s, `check-llvm-unit`: 4s.

Reviewed By: yln, rnk

Differential Revision: https://reviews.llvm.org/D122251