[flang] Make binary->decimal conversion buffer sizes accurate

The binary values that produce the most significant decimal
digits in an exact conversion are those with the least normal
biased exponent (1) and all fractional bits set, not the
least-valued subnormals.  So the binary->decimal conversion
buffer sizes were a little short, and could cause a overrun crash.

Differential revision: https://reviews.llvm.org/D88688