[MC] Leverage constexpr `std::array` in `SubtargetFeature.h`

[jloser]

Summary

Replace C-style array with std::array since std::array<T, N>::operator[] is
constexpr in C++17. This also allows us to replace array_lengthof calls with
member size() function.

[craig.topper]

LGTM

[jloser]

Fix typo in constructor member initialization list

[jloser]

Rebase to see if CI failure was a flake. It looks to be unrelated.

llvm/include/llvm/MC/SubtargetFeature.h

 
 /// Container class for subtarget features.
 /// This is a constexpr reimplementation of a subset of std::bitset. It would be
 /// nice to use std::bitset directly, but it doesn't support constant
 /// initialization.
 class FeatureBitset {
   static_assert((MAX_SUBTARGET_FEATURES % 64) == 0,
                 "Should be a multiple of 64!");
-  // This cannot be a std::array, operator[] is not constexpr until C++17.
-  uint64_t Bits[MAX_SUBTARGET_WORDS] = {};
+  std::array<uint64_t, MAX_SUBTARGET_WORDS> Bits{};
 
 protected:
-  constexpr FeatureBitset(const std::array<uint64_t, MAX_SUBTARGET_WORDS> &B) {
-    for (unsigned I = 0; I != B.size(); ++I)
-      Bits[I] = B[I];
-  }
+  constexpr FeatureBitset(const std::array<uint64_t, MAX_SUBTARGET_WORDS> &B)
+      : Bits{B} {}
 
 public:
   constexpr FeatureBitset() = default;
   constexpr FeatureBitset(std::initializer_list<unsigned> Init) {
     for (auto I : Init)
       set(I);
   }
 
public:
   size_t count() const {
     size_t Count = 0;
     for (auto B : Bits)
       Count += countPopulation(B);
     return Count;
   }
 
   constexpr FeatureBitset &operator^=(const FeatureBitset &RHS) {
-    for (unsigned I = 0, E = array_lengthof(Bits); I != E; ++I) {
+    for (unsigned I = 0, E = Bits.size(); I != E; ++I) {
       Bits[I] ^= RHS.Bits[I];
     }
     return *this;
   }
   constexpr FeatureBitset operator^(const FeatureBitset &RHS) const {
     FeatureBitset Result = *this;
     Result ^= RHS;
     return Result;
   }
 
   constexpr FeatureBitset &operator&=(const FeatureBitset &RHS) {
-    for (unsigned I = 0, E = array_lengthof(Bits); I != E; ++I) {
+    for (unsigned I = 0, E = Bits.size(); I != E; ++I) {
       Bits[I] &= RHS.Bits[I];
     }
     return *this;
   }
   constexpr FeatureBitset operator&(const FeatureBitset &RHS) const {
     FeatureBitset Result = *this;
     Result &= RHS;
     return Result;
   }
 
   constexpr FeatureBitset &operator|=(const FeatureBitset &RHS) {
-    for (unsigned I = 0, E = array_lengthof(Bits); I != E; ++I) {
+    for (unsigned I = 0, E = Bits.size(); I != E; ++I) {
       Bits[I] |= RHS.Bits[I];
     }
     return *this;
   }
   constexpr FeatureBitset operator|(const FeatureBitset &RHS) const {
     FeatureBitset Result = *this;
     Result |= RHS;
     return Result;