[libc] get_table_negative cannot be constexpr

[gchatelet]

Summary

GCC complains about if conditions not being constant expressions.
As-is get_table_negative cannot be marked constexpr.

libc/src/__support/float_to_string.h

 // The formula for the table when i is negative (or zero) is as follows:
 // floor(10^(-9i) * 2^(c_0 - e)) % (10^9 * 2^c_0)
 // Since we know i is always negative, we just take it as unsigned and treat it
 // as negative. We do the same with exponent, while they're both always negative
 // in theory, in practice they're converted to positive for simpler
 // calculations.
 // The formula being used looks more like this:
 // floor(10^(9*(-i)) * 2^(c_0 + (-e))) % (10^9 * 2^c_0)
-constexpr inline cpp::UInt<MID_INT_SIZE>
-get_table_negative(int exponent, size_t i, const size_t constant) {
+inline cpp::UInt<MID_INT_SIZE> get_table_negative(int exponent, size_t i,
+                                                  const size_t constant) {
   constexpr size_t INT_SIZE = 1024;
   int shift_amount = constant - exponent;
   cpp::UInt<INT_SIZE> num(1);
   // const cpp::UInt<INT_SIZE> MOD_SIZE =
   //     (cpp::UInt<INT_SIZE>(1) << constant) * 1000000000;
 
   constexpr uint64_t TEN_EXP_NINE = 1000000000;
   constexpr uint64_t FIVE_EXP_NINE = 1953125;