[ADT] static_assert that SmallDenseMap is instantiated with a power-of-two number of inline buckets.

[jlebar]

Summary

If you try to instantiate it with a non-power-of-two buckets, DenseMap
will assert at runtime (!) if we ever outgrow our inline storage.

I believe using a constexpr function inside of a static_assert is safe
now that we've unsupported MSVC 2013 and GCC < 4.8.

[bkramer]

This makes sense to me.

[jlebar]

Thank you for the review, Benjamin.

llvm/include/llvm/ADT/DenseMap.h

 class SmallDenseMap
     : public DenseMapBase<
           SmallDenseMap<KeyT, ValueT, InlineBuckets, KeyInfoT, BucketT>, KeyT,
           ValueT, KeyInfoT, BucketT> {
   // Lift some types from the dependent base class into this class for
   // simplicity of referring to them.
   typedef DenseMapBase<SmallDenseMap, KeyT, ValueT, KeyInfoT, BucketT> BaseT;
   friend class DenseMapBase<SmallDenseMap, KeyT, ValueT, KeyInfoT, BucketT>;
+  static_assert(isPowerOf2_64(InlineBuckets),
+                "InlineBuckets must be a power of 2.");
 
   unsigned Small : 1;
   unsigned NumEntries : 31;
   unsigned NumTombstones;
 
   struct LargeRep {
     BucketT *Buckets;
     unsigned NumBuckets;