Found in testing against abseil. Two bugs were found:

1) SHIFT_AMOUNT in float_converter<long double> would sometimes be
   negative causing an underflow when passed as the amount to left shift
   by for BigInt.
2) is_lowest_block had an off-by-one because it was adding 1 to the
   block index.

Both are fixed and there are new tests to catch regressions.

https://github.com/llvm/llvm-project/issues/149993

Newer versions of clang now warn about these, so use explicit
conversion instead.

`long double` is haunted on most architectures, but it is especially so on
i386-linux-gnu. While have 80b of significant data, on i386-linux-gnu this type
has 96b of storage.

Fixes for supporting printf family of conversions for `long double` on
i386-linux-gnu. This allows the libc-stdlib-tests and libc_stdio_unittests
ninja target tests to pass on i386-linux-gnu.

Fixes: #110894
Link: #93709
Co-authored-by: Michael Jones <michaelrj@google.com>

Dyadic floats were an existing option for float to string conversion,
but it had become stale. This patch fixes it up as well as adding proper
config options and test support. Due to the test changes this is a
followup to #110759

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.