Bug 1550422 - P00. Fix Windows compilation. r=dmajor

Following the shift in unified build setup following the removal of gfxPrefs.{cpp,c} we hit this error.

Unified builds made this header get included with other files that use multiple inheritance, and clang-cl about the conflicting inheritance models. Local testing suggests clang-cl doesn't need the pragma anyway, so just take it out.

Differential Revision: https://phabricator.services.mozilla.com/D31465

--HG--
extra : moz-landing-system : lando

gfx/2d/unittest/TestBase.h

@@ -9,8 +9,9 @@
 #include <string>
 #include <vector>
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
 // On MSVC otherwise our generic member pointer trick doesn't work.
+// JYA: Do we still need this?
 #  pragma pointers_to_members(full_generality, single_inheritance)
 #endif