fix -Wunused-variable

Summary:
Exposed by the latest clang:
```lang=bash
folly/test/FixedStringTest.cpp:52:36: error: unused variable 's' [-Werror,-Wunused-variable]
  constexpr folly::FixedString<42> s{"hello world"};
                                   ^
folly/gen/test/StringBenchmark.cpp:176:15: error: unused variable 'line' [-Werror,-Wunused-variable]
  StringPiece line(kLine);
              ^
```

Reviewed By: yfeldblum

Differential Revision: D6264853

fbshipit-source-id: 5cf4b0a8c99eaa31c2499746c70ddc49fdd73074

folly/gen/test/StringBenchmark.cpp

@@ -173,7 +173,6 @@ BENCHMARK_RELATIVE(StringUnsplit_Old_ReusedBuffer, iters) {
 
 BENCHMARK_RELATIVE(StringUnsplit_Gen, iters) {
   size_t s = 0;
-  StringPiece line(kLine);
   while (iters--) {
     fbstring joined = from(testStrVector) | unsplit(',');
     s += joined.size();

folly/test/FixedStringTest.cpp

@@ -50,6 +50,7 @@ TEST(FixedStringCtorTest, Default) {
 
 TEST(FixedStringCtorTest, FromLiterals) {
   constexpr folly::FixedString<42> s{"hello world"};
+  static_assert(s[0] == 'h', "");
   constexpr folly::FixedString<11> s2{"hello world"};
   static_assert(s2[0] == 'h', "");
   static_assert(s2[10] == 'd', "");