Fix -Wstring-conversion warnings

Summary:
clang is particular about not treating strings as booleans, but it offers an "out" by using `expr && "string"` specifically for the use cases of `assert` like ours

**Before**
```
assert(!"Should never happen!"); <-- clang warning: -Wstring-conversion
```

**After**
```
assert(false && "Should never happen!");  <-- no clang warning
```

Reviewed By: yfeldblum

Differential Revision: D33796849

fbshipit-source-id: bd48139d0db04d50572e9ada29da777551774f84

folly/ExceptionWrapper-inl.h

@@ -328,9 +328,10 @@ inline exception_wrapper::exception_wrapper(
 namespace exception_wrapper_detail {
 template <class Ex>
 Ex&& dont_slice(Ex&& ex) {
-  assert(typeid(ex) == typeid(std::decay_t<Ex>) ||
-       !"Dynamic and static exception types don't match. Exception would "
-        "be sliced when storing in exception_wrapper.");
+  assert(
+      (typeid(ex) == typeid(std::decay_t<Ex>)) &&
+      "Dynamic and static exception types don't match. Exception would "
+      "be sliced when storing in exception_wrapper.");
   return std::forward<Ex>(ex);
 }
 } // namespace exception_wrapper_detail

folly/FixedString.h

@@ -68,7 +68,7 @@ using FixedStringBase = FixedStringBase_<>;
 // it's testing for fails. In this way, precondition violations are reported
 // at compile-time instead of at runtime.
 [[noreturn]] inline void assertOutOfBounds() {
-  assert(!"Array index out of bounds in BasicFixedString");
+  assert(false && "Array index out of bounds in BasicFixedString");
   throw_exception<std::out_of_range>(
       "Array index out of bounds in BasicFixedString");
 }
@@ -89,7 +89,9 @@ constexpr std::size_t checkOverflowIfDebug(std::size_t i, std::size_t size) {
 
 // Intentionally NOT constexpr. See note above for assertOutOfBounds
 [[noreturn]] inline void assertNotNullTerminated() noexcept {
-  assert(!"Non-null terminated string used to initialize a BasicFixedString");
+  assert(
+      false &&
+      "Non-null terminated string used to initialize a BasicFixedString");
   std::terminate(); // Fail hard, fail fast.
 }