Revert "Merge to upstream r304942."

This reverts commit 83b1388ecd, reversing
changes made to f20819f925.

Test: treehugger
Bug: None

test/std/utilities/function.objects/func.not_fn/not_fn.pass.cpp

@@ -416,7 +416,7 @@ void throws_in_constructor_test()
       ThrowsOnCopy() = default;
       bool operator()() const {
         assert(false);
-#if defined(TEST_COMPILER_C1XX)
+#if defined(_LIBCPP_MSVC)
         __assume(0);
 #else
         __builtin_unreachable();