Implement double underscore names support in __has_attribute

llvm-svn: 151809
diff --git a/clang/docs/LanguageExtensions.html b/clang/docs/LanguageExtensions.html
index be26d62..23aa4fe 100644
--- a/clang/docs/LanguageExtensions.html
+++ b/clang/docs/LanguageExtensions.html
@@ -257,6 +257,11 @@
 </pre>
 </blockquote>
 
+<p>The attribute name can also be specified with a preceding and
+following <code>__</code> (double underscore) to avoid interference from a macro
+with the same name. For instance, <code>__always_inline__</code> can be used
+instead of <code>always_inline</code>.</p>
+
 <!-- ======================================================================= -->
 <h2 id="has_include">Include File Checking Macros</h2>
 <!-- ======================================================================= -->