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.

llvm-svn: 247695
diff --git a/libcxx/test/std/strings/c.strings/cwchar.pass.cpp b/libcxx/test/std/strings/c.strings/cwchar.pass.cpp
index c3d868f..93d2bb1 100644
--- a/libcxx/test/std/strings/c.strings/cwchar.pass.cpp
+++ b/libcxx/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;