Try to make MSVC 2017 happy.

AArch64BaseInfo.h(316): error C3615: constexpr function 'llvm::SysAlias::SysAlias' cannot result in a constant expression
AArch64BaseInfo.h(316): note: failure was caused by call of undefined function or one not declared 'constexpr'
AArch64BaseInfo.h(316): note: see usage of 'llvm::FeatureBitset::FeatureBitset'

llvm-svn: 369851

llvm/include/llvm/MC/SubtargetFeature.h

@@ -50,7 +50,7 @@ protected:
   }
 
 public:
-  FeatureBitset() = default;
+  constexpr FeatureBitset() = default;
   constexpr FeatureBitset(std::initializer_list<unsigned> Init) {
     for (auto I : Init)
       set(I);