[Cfp-interest 3100] Issue: frexp and double-double underflow

[Hubert Tong]

frexp is not specified to have any failure conditions, but it necessarily
will fail to return the specified values for some double-double inputs.

The following program will fail on the second assertion due to underflow in
the less significant component of the double-double value.
#include <assert.h>
#include <math.h>
#include <float.h>
int main() {
  int exp;
  const long double oneplus = 1.L + DBL_TRUE_MIN;
  const long double frac = frexp(oneplus, &exp);
  assert(exp == 1);
  assert(frac * 2 == oneplus);
}
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