[libcxx] Do not implicitly #include assert.h

Users should only get the assert() macros if they explicitly include
them.

Found after switching from the GNU C++ stdlib to the LLVM C++ stdlib.

llvm-svn: 372963
diff --git a/libcxx/test/std/experimental/simd/simd.mem/load.pass.cpp b/libcxx/test/std/experimental/simd/simd.mem/load.pass.cpp
index 1a56161..bd9d563 100644
--- a/libcxx/test/std/experimental/simd/simd.mem/load.pass.cpp
+++ b/libcxx/test/std/experimental/simd/simd.mem/load.pass.cpp
@@ -15,6 +15,7 @@
 
 #include <experimental/simd>
 #include <cstdint>
+#include <cassert>
 
 #include "test_macros.h"