Change CountDecimalDigit32() to simple implementation

It is simple and pure C++. And it is found in performance test that it
is even faster than the original version, due to distribution of n. But
the performance gain is not obvious in RapidJSON.

include/rapidjson/internal/dtoa.h

@@ -218,27 +218,17 @@ inline void GrisuRound(char* buffer, int len, uint64_t delta, uint64_t rest, uin
 }
 
 inline unsigned CountDecimalDigit32(uint32_t n) {
-	static const uint32_t powers_of_10[] = {
-		0,
-		10,
-		100,
-		1000,
-		10000,
-		100000,
-		1000000,
-		10000000,
-		100000000,
-		1000000000
-	};
-
-#ifdef _MSC_VER
-	unsigned long i = 0;
-	_BitScanReverse(&i, n | 1);
-	uint32_t t = (i + 1) * 1233 >> 12;
-#elif __GNUC__
-	uint32_t t = (32 - __builtin_clz(n | 1)) * 1233 >> 12;
-#endif
-	return t - (n < powers_of_10[t]) + 1;
+	// Simple pure C++ implementation was faster than __builtin_clz version in this situation.
+	if (n < 10) return 1;
+	if (n < 100) return 2;
+	if (n < 1000) return 3;
+	if (n < 10000) return 4;
+	if (n < 100000) return 5;
+	if (n < 1000000) return 6;
+	if (n < 10000000) return 7;
+	if (n < 100000000) return 8;
+	if (n < 1000000000) return 9;
+	return 10;
 }
 
 inline void DigitGen(const DiyFp& W, const DiyFp& Mp, uint64_t delta, char* buffer, int* len, int* K) {