Support the tls_model attribute (PR9788)

This adds support for the tls_model attribute. This allows the user to
choose a TLS model that is better than what LLVM would select by
default. For example, a variable might be declared as:

  __thread int x __attribute__((tls_model("initial-exec")));

if it will not be used in a shared library that is dlopen'ed.

This depends on LLVM r159077.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159078 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/ReleaseNotes.html b/docs/ReleaseNotes.html
index 2943773..7bca3bf 100644
--- a/docs/ReleaseNotes.html
+++ b/docs/ReleaseNotes.html
@@ -123,6 +123,20 @@
   flag.</li>
 </ul>
 
+<h4 id="tlsmodel">Support for <code>tls_model</code> attribute</h4>
+
+<p>Clang now supports the <code>tls_model</code> attribute, allowing code that
+uses thread-local storage to explicitly select which model to use. The available
+models are <code>"global-dynamic"</code>, <code>"local-dynamic"</code>,
+<code>"initial-exec"</code> and <code>"local-exec"</code>. See
+<a href="http://www.akkadia.org/drepper/tls.pdf">ELF Handling For Thread-Local
+ Storage</a> for more information.</p>
+
+<p>The compiler is free to choose a different model if the specified model is not
+supported by the target, or if the compiler determines that a more specific
+model can be used.
+</p>
+
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 <h3 id="newflags">New Compiler Flags</h3>
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