Bug 1612536 - Make array iterator getters constexpr r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D61366

--HG--
extra : moz-landing-system : lando

mfbt/Array.h

@@ -64,11 +64,11 @@ class Array {
 
   // Methods for range-based for loops.
   iterator begin() { return mArr; }
-  const_iterator begin() const { return mArr; }
-  const_iterator cbegin() const { return begin(); }
+  constexpr const_iterator begin() const { return mArr; }
+  constexpr const_iterator cbegin() const { return begin(); }
   iterator end() { return mArr + Length; }
-  const_iterator end() const { return mArr + Length; }
-  const_iterator cend() const { return end(); }
+  constexpr const_iterator end() const { return mArr + Length; }
+  constexpr const_iterator cend() const { return end(); }
 
   // Methods for reverse iterating.
   reverse_iterator rbegin() { return reverse_iterator(end()); }