Change initialization of mbstate_t objects in tests from '= {0}' to '= {}', which does the same thing, w/o having clang and gcc warn with -Wall.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@247695 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/strings/c.strings/cwchar.pass.cpp

@@ -30,7 +30,7 @@
 
 int main()
 {
-    std::mbstate_t mb = {0};
+    std::mbstate_t mb = {};
     std::size_t s = 0;
     std::tm *tm = 0;
     std::wint_t w = 0;