Fix integer overflow in ConcurrentHashtTableByPtr

This change fixes a case where there was a potential integer overflow
when multiplying two 32-bit values and assigning the result to a 64-bit
value.

The potential overflow has been present since https://reviews.llvm.org/D132455
was re-landed. The initial version of the patch defined MaxBucketSize and
NumberOfBuckets as size_t (which I guess would still be a problem for some
targets). When the patch was re-landed they were defined as uint32_t, making
the calculation of ExtHashMask subject to overflow.

Differential Revision: https://reviews.llvm.org/D158117

Author: Andy Kaylor

llvm/include/llvm/ADT/ConcurrentHashtable.h

@@ -154,7 +154,7 @@ class ConcurrentHashTableByPtr {
     MaxBucketSize = 1Ull << (std::min((size_t)31, LeadingZerosNumber));
 
     // Calculate mask for extended hash bits.
-    ExtHashMask = (NumberOfBuckets * MaxBucketSize) - 1;
+    ExtHashMask = (uint64_t)NumberOfBuckets * MaxBucketSize - 1;
   }
 
   virtual ~ConcurrentHashTableByPtr() {