Include the rustc_apfloat crate in COPYRIGHT

The rustc_apfloat crate is a port of LLVM's APFloat code and was
introduced in #43554. At that
time, LLVM was licensed under the UIUC license.

Since that time, contributors have made changes to the code without
being advised of the UIUC licensing resulting in the code base being a
mixture of UIUC licensed code and Apache 2/MIT licensed code.

Going forward, the compiler team will be asking contributors to license
their contributions under all of Apache 2/MIT/UIUC licenses so that,
eventually, either we can relicense the crate as simply UIUC licensed or
create a new port that is Apache 2/MIT licensed which can use the
changes contributed on top of the UIUC base.

Author: wesleywiser

COPYRIGHT

@@ -110,3 +110,57 @@ their own copyright notices and license terms:
     NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE
     USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
     OF SUCH DAMAGE.
+
+* The rustc_apfloat crate, located in compiler/rustc_apfloat, is based
+  on the APFloat library from a version of LLVM licensed under the UIUC
+  license. Contributions to the rustc_apfloat crate are licensed either
+  under the standard Rust licensing (Apache License, Version 2.0 or the
+  MIT license, at your option) or under the Apache License, Version 2.0,
+  the MIT license or the UIUC license, at your option.
+
+    Copyright (c) 2003-2013 University of Illinois at
+    Urbana-Champaign.  All rights reserved.
+
+    Developed by:
+
+        LLVM Team
+
+        University of Illinois at Urbana-Champaign
+
+        http://llvm.org
+
+    Permission is hereby granted, free of charge, to any
+    person obtaining a copy of this software and associated
+    documentation files (the "Software"), to deal with the
+    Software without restriction, including without
+    limitation the rights to use, copy, modify, merge,
+    publish, distribute, sublicense, and/or sell copies of
+    the Software, and to permit persons to whom the Software
+    is furnished to do so, subject to the following
+    conditions:
+
+        * Redistributions of source code must retain the
+          above copyright notice, this list of conditions
+          and the following disclaimers.
+
+        * Redistributions in binary form must reproduce the
+          above copyright notice, this list of conditions
+          and the following disclaimers in the documentation
+          and/or other materials provided with the
+          distribution.
+
+        * Neither the names of the LLVM Team, University of
+          Illinois at Urbana-Champaign, nor the names of its
+          contributors may be used to endorse or promote
+          products derived from this Software without
+          specific prior written permission.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+    ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+    TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+    PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT
+    SHALL THE CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE
+    FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+    ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT
+    OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+    OTHER DEALINGS WITH THE SOFTWARE.