Fix "use of" uninitialized memory in benchmark.

An argument to DoNotOptimize was not fully initialized, which caused
msan to complain.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@346808 91177308-0d34-0410-b5e6-96231b3b80d8

benchmarks/function.bench.cpp

@@ -48,7 +48,7 @@ struct AllOpacity : EnumValuesAsTuple<AllOpacity, Opacity, 2> {
 
 struct S {
   int function() const { return 0; }
-  int field;
+  int field = 0;
 };
 
 int FunctionWithS(const S*) { return 0; }