Fix memory error in the implementation of IndexSubset. (#27850)

Author: bgogul

include/swift/AST/IndexSubset.h

@@ -57,6 +57,10 @@ class IndexSubset : public llvm::FoldingSetNode {
   /// The number of bit words in the index subset.
   unsigned numBitWords;
 
+  static unsigned getNumBytesNeededForCapacity(unsigned capacity) {
+    return getNumBitWordsNeededForCapacity(capacity) * bitWordSize;
+  }
+
   BitWord *getBitWordsData() {
     return reinterpret_cast<BitWord *>(this + 1);
   }

lib/AST/ASTContext.cpp

@@ -4822,7 +4822,7 @@ IndexSubset::get(ASTContext &ctx, const SmallBitVector &indices) {
   if (existing)
     return existing;
   auto sizeToAlloc = sizeof(IndexSubset) +
-      getNumBitWordsNeededForCapacity(capacity);
+      getNumBytesNeededForCapacity(capacity);
   auto *buf = reinterpret_cast<IndexSubset *>(
       ctx.Allocate(sizeToAlloc, alignof(IndexSubset)));
   auto *newNode = new (buf) IndexSubset(indices);