Fix MSVC "32-bit shift implicitly converted to 64 bits" warning. NFC

NumBits should be less than 20 so using an unsigned instead of size_t should be OK

Author: RKSimon

llvm/lib/Support/TrieRawHashMap.cpp

@@ -79,7 +79,7 @@ class TrieSubtrie final
 
   static constexpr size_t sizeToAlloc(unsigned NumBits) {
     assert(NumBits < 20 && "Tries should have fewer than ~1M slots");
-    size_t Count = 1u << NumBits;
+    unsigned Count = 1u << NumBits;
     return totalSizeToAlloc<LazyAtomicPointer<TrieNode>>(Count);
   }