Expose __builtin_bswap16.

GCC has always supported this on PowerPC and 4.8 supports it on all platforms,
so it's a good idea to expose it in clang too. LLVM supports this on all targets.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@165362 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/ExprConstant.cpp b/lib/AST/ExprConstant.cpp
index e21c83b..323a174 100644
--- a/lib/AST/ExprConstant.cpp
+++ b/lib/AST/ExprConstant.cpp
@@ -4296,6 +4296,7 @@
     return Error(E);
   }
 
+  case Builtin::BI__builtin_bswap16:
   case Builtin::BI__builtin_bswap32:
   case Builtin::BI__builtin_bswap64: {
     APSInt Val;