Remove direct use of _builtin_clz.

A previous CL introduced _builtin_clz in zippy.cc. This is a GCC / Clang
intrinsic, and is not supported in Visual Studio. The rest of the
project uses bit manipulation intrinsics via the functions in Bits::,
which are stubbed out for the open source build in
zippy-stubs-internal.h.

This CL extracts Bits::Log2FloorNonZero() out of Bits::Log2Floor() in
the stubbed version of Bits, adds assertions to the Bits::*NonZero()
functions in the stubs, and converts _builtin_clz to a
Bits::Log2FloorNonZero() call.

The latter part is not obvious. A mathematical proof of correctness is
outlined in added comments. An empirical proof is available at
https://godbolt.org/z/mPKWmh -- CalculateTableSizeOld(), which is the
current code, compiles to the same assembly on Clang as
CalculateTableSizeNew1(), which is the bigger jump in the proof.
CalculateTableSizeNew2() is a fairly obvious transformation from
CalculateTableSizeNew1(), and results in slightly better assembly on all
supported compilers.

Two benchmark runs with the same arguments as the original CL only
showed differences in completely disjoint tests, suggesting that the
differences are pure noise.

snappy.cc

@@ -429,7 +429,9 @@ uint32 CalculateTableSize(uint32 input_size) {
   if (input_size < 256) {
     return 256;
   }
-  return 1u << (32 - __builtin_clz(input_size - 1));
+  // This is equivalent to Log2Ceiling(input_size), assuming input_size > 1.
+  // 2 << Log2Floor(x - 1) is equivalent to 1 << (1 + Log2Floor(x - 1)).
+  return 2u << Bits::Log2Floor(input_size - 1);
 }
 }  // namespace