Enable testing with _LIBCPP_DEBUG and fix bad assertions in string_view.

llvm-svn: 229698
diff --git a/libcxx/www/lit_usage.html b/libcxx/www/lit_usage.html
index 645143f..77179f2 100644
--- a/libcxx/www/lit_usage.html
+++ b/libcxx/www/lit_usage.html
@@ -175,6 +175,15 @@
 </p>
 
 <p>
+<h3 class="lit-option">debug_level=&lt;level&gt;</h3>
+<blockquote class="lit-option-desc">
+<b>Values: </b><code>0, 1</code></br>
+Enable the use of debug mode. Level 0 enables assertions and level 1 enables
+assertions and debugging of iterator misuse.
+</blockquote>
+</p>
+
+<p>
 <h3 class="lit-option">use_sanitizer=&lt;sanitizer name&gt;</h3>
 <blockquote class="lit-option-desc">
 <b>Values: </b><code>Memory, MemoryWithOrigins, Address, Undefined</code></br>