fix literal_string under gcc

Summary:
Fixes this compile error under gcc:
```
folly/test/UtilityTest.cpp: In member function ‘virtual void UtilityTest_literal_string_Test::TestBody()’:
folly/test/UtilityTest.cpp:195:57: error: ‘constexpr folly::literal_string<C, N>::literal_string(const C (&)[N]) [with C = char; long unsigned int N = 12]’ called in a constant expression
  195 |   constexpr auto s = folly::literal_string{"hello world"};
      |                                                         ^
In file included from folly/test/UtilityTest.cpp:17:
folly/Utility.h:279:34: note: ‘constexpr folly::literal_string<C, N>::literal_string(const C (&)[N]) [with C = char; long unsigned int N = 12]’ is not usable as a ‘constexpr’ function because:
  279 |   FOLLY_CONSTEVAL /* implicit */ literal_string(C const (&buf)[N]) noexcept {
      |                                  ^~~~~~~~~~~~~~
folly/Utility.h:279:34: error: member ‘folly::literal_string<char, 12>::buffer’ must be initialized by mem-initializer in ‘constexpr’ constructor
folly/Utility.h:277:5: note: declared here
  277 |   C buffer[N];
      |     ^~~~~~
```

And add the corresponding test to the cmake build.

Reviewed By: DenisYaroshevskiy

Differential Revision: D62217752

fbshipit-source-id: bf2b8892b37848d3e765f6a9752d7ba7a87d5a93

CMakeLists.txt

@@ -1156,6 +1156,7 @@ if (BUILD_TESTS OR BUILD_BENCHMARKS)
       TEST unit_test SOURCES UnitTest.cpp
       BENCHMARK uri_benchmark SOURCES UriBenchmark.cpp
       TEST uri_test SOURCES UriTest.cpp
+      TEST utility_test SOURCES UtilityTest.cpp
       TEST varint_test SOURCES VarintTest.cpp
 
     DIRECTORY testing/test/

folly/Utility.h

@@ -274,7 +274,7 @@ inline constexpr identity_fn identity{};
 ///   }
 template <typename C, std::size_t N>
 struct literal_string {
-  C buffer[N];
+  C buffer[N] = {};
 
   FOLLY_CONSTEVAL /* implicit */ literal_string(C const (&buf)[N]) noexcept {
     for (std::size_t i = 0; i < N; ++i) {

folly/test/UtilityTest.cpp

@@ -191,6 +191,8 @@ TEST_F(UtilityTest, forward_like) {
   // folly::forward_like<const int&>(1);
 }
 
+static_assert(!std::is_default_constructible_v<folly::literal_string<char, 1>>);
+
 TEST_F(UtilityTest, literal_string) {
   constexpr auto s = folly::literal_string{"hello world"};
   EXPECT_STREQ("hello world", s.c_str());