[4.2] Updates to Floating-point printing code (SwiftDtoa.cpp) (#16178)

[stephentyrone]

Cherry picking @tbkka 's change from master for 4.2:

This collects a number of changes I've been testing over the
last month.

• Bug fix: The single-precision float formatter did not always
  round the last digit even in cases where there were two
  possible outputs that were otherwise equally good.

• Algorithm simplification: The condition for determining
  whether to widen or narrow the interval was more complex than
  necessary. I now simply widen the interval for all even
  significands.

• Code simplification: The single-precision float formatter now uses fewer
  64-bit features. This eliminated some 32-bit vs. 64-bit conditionals in
  exchange for a minor loss of performance (~2%).

• Minor performance tweaks: Steve Canon pointed out a few places
  where I could avoid some extraneous arithmetic.

I've also rewritten a lot of comments to try to make the exposition
clearer.

The earlier testing regime focused on testing from first
principles. For example, I verified accuracy by feeding the
result back into the C library strtof, strtod, etc. and
checking round-trip exactness. Unfortunately, this approach
requires many checks for each value, limiting test performance.
It's also difficult to validate last-digit rounding.

For this round of updates, I've instead compared the digit
decompositions to other popular algorithms:

• David M. Gay's gdtoa library is a robust and well-tested
  implementation based on Dragon4. It supports all formats, but
  is slow. (netlib.org/fp)
• Grisu3 supports Float and Double. It is fast but incomplete,
  failing on about 1% of all inputs.
  (github.com/google/double-conversion)
• Errol4 is fast and complete but only supports Double. The
  repository includes an implementation of the enumeration
  algorithm described in the Errol paper.
  (github.com/marcandrysco/errol)

The exact tests varied by format:

• Float: SwiftDtoa now generates the exact same digits as gdtoa
  for every single-precision Float.

• Double: Testing against Grisu3 (with fallback to Errol4 when
  Grisu3 failed) greatly improved test performance. This
  allowed me to test 100 trillion (10^14) randomly-selected
  doubles in a reasonable amount of time. I also checked all
  values generated by the Errol enumeration algorithm.

• Float80: I compared the Float80 output to the gdtoa library
  because neither Grisu3 nor Errol4 yet supports 80-bit extended
  precision. All values generated by the Errol enumeration
  algorithm have been checked, as well as several billion
  randomly-selected values.

[stephentyrone]

@swift-ci Please smoke test

[airspeedswift]

I think you may need full test for 4.2 PRs

[airspeedswift]

@swift-ci please test

[airspeedswift]

@swift-ci please smoke test Linux platform

[stephentyrone]

@swift-ci please smoke test Linux

[stephentyrone]

@airspeedswift OK to merge this?

[airspeedswift]

Yep