[flang] Fix x87 binary->decimal

Summary:
Fix decimal formatting of 80-bit x87 values; the calculation ofnearest neighbor values failed to account for the explicitmost significant bit in that format.

Replace MultiplyByRounded with MultiplyBy in binary->decimal conversions,
since rounding won't happen and the name was misleading; then remove
dead code, and migrate LoseLeastSignificantDigit() from one source file
to another where it's still needed.

Reviewers: tskeith, sscalpone, jdoerfert, DavidTruby

Reviewed By: tskeith

Subscribers: llvm-commits, flang-commits

Tags: #flang, #llvm

Differential Revision: https://reviews.llvm.org/D79345