System: Add SwapByteOrder and update Support/MathExtras.h to use it.

This time correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116247 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/unittests/CMakeLists.txt b/unittests/CMakeLists.txt
index 43dfd23..8a5a367 100644
--- a/unittests/CMakeLists.txt
+++ b/unittests/CMakeLists.txt
@@ -95,6 +95,7 @@
   Support/raw_ostream_test.cpp
   Support/RegexTest.cpp
   Support/System.cpp
+  Support/SwapByteOrderTest.cpp
   Support/TypeBuilderTest.cpp
   Support/ValueHandleTest.cpp
   )