Bug 1897752 - Make EnumeratedArray constructor constexpr.

MANUAL PUSH: Trivial base toolchains fix CLOSED TREE.

mfbt/EnumeratedArray.h

@@ -55,7 +55,7 @@ class EnumeratedArray {
   ArrayType mArray;
 
  public:
-  EnumeratedArray() = default;
+  constexpr EnumeratedArray() = default;
 
   template <typename... Args>
   MOZ_IMPLICIT constexpr EnumeratedArray(Args&&... aArgs)