fix warning violations of array-bounds in small_vector

Summary:
Fixes or works around the following warning violations:
```
In file included from folly/small_vector.h:17,
                 from folly/container/test/small_vector_test.cpp:17:
In lambda function,
    inlined from ‘void folly::detail::populateMemForward(T*, std::size_t, const Function&) [with T = int; Function = folly::small_vector<int>::constructImpl(size_type, const value_type&, std::true_type)::<lambda(void*)>]’ at folly/container/small_vector.h:229:9,
    inlined from ‘void folly::small_vector<T, M, P>::doConstruct(size_type, InitFunc&&) [with InitFunc = folly::small_vector<int>::constructImpl(size_type, const value_type&, std::true_type)::<lambda(void*)>; Value = int; long unsigned int RequestedMaxInline = 1; Policy = void]’ at folly/container/small_vector.h:1164:33,
    inlined from ‘void folly::small_vector<T, M, P>::constructImpl(size_type, const value_type&, std::true_type) [with Value = int; long unsigned int RequestedMaxInline = 1; Policy = void]’ at folly/container/small_vector.h:1172:16,
    inlined from ‘folly::small_vector<T, M, P>::small_vector(Arg, Arg) [with Arg = int; Value = int; long unsigned int RequestedMaxInline = 1; Policy = void]’ at folly/container/small_vector.h:575:18,
    inlined from ‘virtual void smallVector_InsertTrivial_Test::TestBody()’ at folly/container/test/small_vector_test.cpp:406:40:
folly/container/small_vector.h:1172:35: warning: array subscript 4 is outside array bounds of ‘folly::small_vector<int> [1]’ [-Warray-bounds=]
 1172 |     doConstruct(n, [&](void* p) { new (p) value_type(val); });
      |                                   ^~~~~~~~~~~~~~~~~~~~~~~
```

Reviewed By: ilvokhin

Differential Revision: D68424534

fbshipit-source-id: 9ba06273d38b6008fa4e759fdcf4e40859567c4c

third-party/folly/src/folly/container/small_vector.h

@@ -560,11 +560,17 @@ class small_vector
   }
 
   explicit small_vector(size_type n) {
+    FOLLY_PUSH_WARNING
+    FOLLY_GCC_DISABLE_WARNING("-Warray-bounds")
     doConstruct(n, [&](void* p) { new (p) value_type(); });
+    FOLLY_POP_WARNING
   }
 
   small_vector(size_type n, value_type const& t) {
+    FOLLY_PUSH_WARNING
+    FOLLY_GCC_DISABLE_WARNING("-Warray-bounds")
     doConstruct(n, [&](void* p) { new (p) value_type(t); });
+    FOLLY_POP_WARNING
   }
 
   template <class Arg>
@@ -773,7 +779,10 @@ class small_vector
 
   void resize(size_type sz, value_type const& v) {
     if (sz < size()) {
+      FOLLY_PUSH_WARNING
+      FOLLY_GCC_DISABLE_WARNING("-Warray-bounds")
       erase(begin() + sz, end());
+      FOLLY_POP_WARNING
       return;
     }
     auto extra = sz - size();
@@ -1169,7 +1178,10 @@ class small_vector
   // The true_type means we should forward to the size_t,value_type
   // overload.
   void constructImpl(size_type n, value_type const& val, std::true_type) {
+    FOLLY_PUSH_WARNING
+    FOLLY_GCC_DISABLE_WARNING("-Warray-bounds")
     doConstruct(n, [&](void* p) { new (p) value_type(val); });
+    FOLLY_POP_WARNING
   }
 
   /*