mention the non-standard builtin macros we support.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@68935 91177308-0d34-0410-b5e6-96231b3b80d8

docs/LanguageExtensions.html

@@ -19,6 +19,7 @@ td {
 
 <ul>
 <li><a href="#intro">Introduction</a></li>
+<li><a href="#builtinmacros">Builtin Macros</a></li>
 <li><a href="#vectors">Vectors and Extended Vectors</a></li>
 <li><a href="#blocks">Blocks</a></li>
 <li><a href="#overloading-in-c">Function Overloading in C</a></li>
@@ -49,6 +50,12 @@ range of GCC extensions.  Please see the <a
 href="http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html">GCC manual</a> for
 more information on these extensions.</p>
 
+<!-- ======================================================================= -->
+<h2 id="builtinmacros">Builtin Macros</h2>
+<!-- ======================================================================= -->
+
+<p>__BASE_FILE__, __INCLUDE_LEVEL__, __TIMESTAMP__, __COUNTER__</p>
+
 <!-- ======================================================================= -->
 <h2 id="vectors">Vectors and Extended Vectors</h2>
 <!-- ======================================================================= -->