[libcxx] Tired of colorless compile errors? Enable color diagnostics today!

Summary:
This patch adds a lit option to enable color diagnostics when either `--param=color_diagnostics` is passed to LIT or `LIBCXX_COLOR_DIAGNOSTICS` is present in the environment.

My only concern with this patch is that GCC and Clang take different flags and that only GCC 4.9 and greater support `-fdiagnostics-color=always`

Does anybody have objections to this going in?

Reviewers: jroelofs, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7729

llvm-svn: 229707
diff --git a/libcxx/www/lit_usage.html b/libcxx/www/lit_usage.html
index 77179f2..c6bfbae 100644
--- a/libcxx/www/lit_usage.html
+++ b/libcxx/www/lit_usage.html
@@ -192,6 +192,16 @@
 </blockquote>
 </p>
 
+<p>
+<h3 class="lit-option">color_diagnostics</h3>
+<blockquote class="lit-option-desc">
+Enable the use of colorized compile diagnostics. If the
+<code>color_diagnostics</code> option is specified or the enviroment variable
+<code>LIBCXX_COLOR_DIAGNOSTICS</code> is present then color diagnostics will be
+enabled.
+</blockquote>
+</p>
+
 </div>
 </body>
 </html>