[libc++][test] Remove Clang <= 3.7 workaround in is_default_constructible test

[jloser]

Summary

Clang 3.7 and below is not actively used or supported in the test suite now, so
remove the workaround in the test.

[Mordante]

LGTM, thanks! The Apple failures are the one I fixed earlier today.

[jloser]

Rebase to fix CI error with modules build being broken. If this passes CI, I'll land it.

libcxx/test/std/utilities/meta/meta.unary/meta.unary.prop/is_default_constructible.pass.cpp

int main(int, char**)
     test_is_not_default_constructible<char[]>();
     test_is_not_default_constructible<char[][3]>();
 
     test_is_not_default_constructible<Abstract>();
     test_is_not_default_constructible<NoDefaultConstructor>();
 #if TEST_STD_VER >= 11
     test_is_not_default_constructible<B>();
     test_is_not_default_constructible<int&&>();
-
-// TODO: Remove this workaround once Clang <= 3.7 are no longer used regularly.
-// In those compiler versions the __is_constructible builtin gives the wrong
-// results for abominable function types.
-#if (defined(TEST_APPLE_CLANG_VER) && TEST_APPLE_CLANG_VER < 703) \
- || (defined(TEST_CLANG_VER) && TEST_CLANG_VER < 308)
-#define WORKAROUND_CLANG_BUG
-#endif
-#if !defined(WORKAROUND_CLANG_BUG)
     test_is_not_default_constructible<void()>();
     test_is_not_default_constructible<void() const> ();
     test_is_not_default_constructible<void() volatile> ();
     test_is_not_default_constructible<void() &> ();
     test_is_not_default_constructible<void() &&> ();
 #endif
-#endif
 
   return 0;
 }