Let IPAddressV4::bitCount and IPAddressV6::bitCount be constexpr

Summary: [Folly] Let `IPAddressV4::bitCount` and `IPAddressV6::bitCount` be `constexpr`.

Reviewed By: meyering

Differential Revision: D5589954

fbshipit-source-id: f4208efd62e71f47a7c87bdc8ddd421f93413c29

folly/IPAddressV4.h

@@ -140,7 +140,7 @@ class IPAddressV4 {
    * @see IPAddress#bitCount
    * @returns 32
    */
-  static size_t bitCount() {
+  static constexpr size_t bitCount() {
     return 32;
   }
 

folly/IPAddressV6.h

@@ -173,7 +173,7 @@ class IPAddressV6 {
    * @see IPAddress#bitCount
    * @returns 128
    */
-  static size_t bitCount() {
+  static constexpr size_t bitCount() {
     return 128;
   }