APFloat/x87: Fix string conversion for "unnormal" values (pr35860)

Summary:
Unnormal values are a feature of some very old x87 processors. We handle
them correctly for the most part -- the only exception was an unnormal
value whose significand happened to be zero. In this case the APFloat
was still initialized as normal number (category = fcNormal), but a
subsequent toString operation would assert because the math would
produce nonsensical values for the zero significand.

During review, it was decided that the correct way to fix this is to
treat all unnormal values as NaNs (as that is what any >=386 processor
will do).

The issue was discovered because LLDB would crash when trying to print
some "long double" values.

Reviewers: skatkov, scanon, gottesmm

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D41868

llvm-svn: 331884