Change default # of digits for APFloat::toString

This is a re-commit of r189442; I'll follow up with clang changes.

The previous default was almost, but not quite enough digits to
represent a floating-point value in a manner which preserves the
representation when it's read back in.  The larger default is much
less confusing.

I spent some time looking into printing exactly the right number of
digits if a precision isn't specified, but it's kind of complicated,
and I'm not really sure I understand what APFloat::toString is supposed
to output for FormatPrecision != 0 (or maybe the current API specification
is just silly, not sure which).  I have a WIP patch if anyone is interested.

llvm-svn: 189624