Bug 1022050 - Disable the SEH block used in TestPoisonArea.cpp on clang-cl until it grows SEH support; r=froydnj

mfbt/tests/TestPoisonArea.cpp

@@ -400,14 +400,14 @@ IsBadExecPtr(uintptr_t ptr)
 {
   BOOL ret = false;
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && !defined(__clang__)
   __try {
     JumpTo(ptr);
   } __except (EXCEPTION_EXECUTE_HANDLER) {
     ret = true;
   }
 #else
-  printf("INFO | exec test not supported on MinGW build\n");
+  printf("INFO | exec test not supported on MinGW or clang-cl builds\n");
   // We do our best
   ret = IsBadReadPtr((const void*)ptr, 1);
 #endif