Enable -Wstring-conversion in folly/PACKAGE +1

Summary:
This diff enables compilation warning flags for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it work; test errors are spurious.**

Differential Revision: D92431440

fbshipit-source-id: de0b00835d823253acf6070b21a5a4ecf9d907d8

Author: r-barnes

folly/test/AtomicHashArrayTest.cpp

@@ -113,7 +113,7 @@ void testMap() {
     ref.insert(e);
     EXPECT_EQ(ref.size(), arr->size());
     if (ret.first == arr->end()) {
-      EXPECT_FALSE("AHA should not have run out of space.");
+      EXPECT_TRUE(false && "AHA should not have run out of space.");
       continue;
     }
     EXPECT_EQ(e.first, ret.first->first);
@@ -140,7 +140,7 @@ void testMap() {
   for (const auto& e : ref) {
     auto ret = arr->find(e.first);
     if (ret == arr->end()) {
-      EXPECT_FALSE("Key was not in AHA");
+      EXPECT_TRUE(false && "Key was not in AHA");
       continue;
     }
     EXPECT_EQ(e.first, ret->first);

folly/test/AtomicHashMapTest.cpp

@@ -544,7 +544,7 @@ void raceIterateThread(std::atomic<bool>* shouldIterate, size_t maxSize) {
     for (; it != end; ++it) {
       ++count;
       if (count > maxSize) {
-        EXPECT_FALSE("Infinite loop in iterator.");
+        EXPECT_TRUE(false && "Infinite loop in iterator.");
         return;
       }
     }