Basic: set the correct storage class for the alignment

`cl` objects to the initialization of the templated type differing in storage
class due to the indication of `constexpr`.  `constexpr` does not give the value
itself a `const` storage class.  However, because the value is not initialized
inline, it does not like the `constexpr` attribute.  Apply the `constexpr` only
on clang, and correct the storage to `const`.

Author: compnerd

include/swift/Basic/ImmutablePointerSet.h

@@ -349,7 +349,10 @@ ImmutablePointerSet<T> ImmutablePointerSetFactory<T>::EmptyPtrSet =
     ImmutablePointerSet<T>(nullptr, {});
 
 template <typename T>
-constexpr unsigned ImmutablePointerSetFactory<T>::AllocAlignment =
+#if !defined(_MSC_VER) || defined(__clang__)
+constexpr
+#endif
+const unsigned ImmutablePointerSetFactory<T>::AllocAlignment =
     (alignof(PtrSet) > alignof(PtrTy)) ? alignof(PtrSet) : alignof(PtrTy);
 
 } // end swift namespace