Add information about unit tests to the testing guide

[carwil]

Thanks for posting this! Always good to see doc updates.
Bit of _very_ opinionated bike-shedding. Content otherwise LGTM!

docs/TestingGuide.rst

101

This is incredibly picky, but I'd prefer an 'and' here.

104

ditto.

110

ditto (perhaps with a capital R, as well).

[michaelplatings]

Addressed @carwil's comments

[carwil]

Thanks for the changes! LGTM. Approved.

docs/TestingGuide.rst

 
 In order to use the LLVM testing infrastructure, you will need all of the
 software required to build LLVM, as well as `Python <http://python.org>`_ 2.7 or
 later.
 
 LLVM Testing Infrastructure Organization
 ========================================
 
-The LLVM testing infrastructure contains two major categories of tests:
-regression tests and whole programs. The regression tests are contained
-inside the LLVM repository itself under ``llvm/test`` and are expected
-to always pass -- they should be run before every commit.
+The LLVM testing infrastructure contains three major categories of tests:
+unit tests, regression tests and whole programs. The unit tests and regression
+tests are contained inside the LLVM repository itself under ``llvm/unittests``
+and ``llvm/test`` respectively and are expected to always pass -- they should be
+run before every commit.
 
 The whole programs tests are referred to as the "LLVM test suite" (or
 "test-suite") and are in the ``test-suite`` module in subversion. For
 historical reasons, these tests are also referred to as the "nightly
 tests" in places, which is less ambiguous than "test-suite" and remains
 in use although we run them much more often than nightly.
 
+Unit tests
+----------
+
+Unit tests are written using `Google Test <https://github.com/google/googletest/blob/master/googletest/docs/primer.md>`_
+and `Google Mock <https://github.com/google/googletest/blob/master/googlemock/docs/ForDummies.md>`_
+and are located in the ``llvm/unittests`` directory.
+
 Regression tests
 ----------------
 
 The regression tests are small pieces of code that test a specific
 feature of LLVM or trigger a specific bug in LLVM. The language they are
 written in depends on the part of LLVM being tested. These tests are driven by
 the :doc:`Lit <CommandGuide/lit>` testing tool (which is part of LLVM), and
 are located in the ``llvm/test`` directory.

 These tests are compiled and run under a debugger. The debugger output
 is checked to validate of debugging information. See README.txt in the
 test suite for more information . This test suite is located in the
 ``debuginfo-tests`` Subversion module.
 
 Quick start
 ===========
 
-The tests are located in two separate Subversion modules. The
-regressions tests are in the main "llvm" module under the directory
-``llvm/test`` (so you get these tests for free with the main LLVM tree).
-Use ``make check-all`` to run the regression tests after building LLVM.
+The tests are located in two separate Subversion modules. The unit &
+regression tests are in the main "llvm" module under the directories
+``llvm/unittests`` and ``llvm/test`` (so you get these tests for free with the
+main LLVM tree). Use ``make check-all`` to run the unit & regression tests after
+building LLVM.
 
 The ``test-suite`` module contains more comprehensive tests including whole C
 and C++ programs. See the :doc:`TestSuiteGuide` for details.
 
-Regression tests
-----------------
+Unit & regression tests
+-----------------------
+
+To run all of the LLVM unit tests use the check-llvm-unit target:
+
+.. code-block:: bash
+
+    % make check-llvm-unit
 
 To run all of the LLVM regression tests use the check-llvm target:
 
 .. code-block:: bash
 
     % make check-llvm
 
 If you have `Clang <http://clang.llvm.org/>`_ checked out and built, you