[analyzer] Set concrete offset bindings to UnknownVal when processing symbolic offset binding, even if no bindings are present.

This addresses an undefined value false positive from concreteOffsetBindingIsInvalidatedBySymbolicOffsetAssignment.

Fixes PR14877; radar://12991168.
Fix a bug in fast-math fadd/fsub simplification.

The problem is that the code mistakenly took for granted that following constructor
is able to create an APFloat from a *SIGNED* integer:

  APFloat::APFloat(const fltSemantics &ourSemantics, integerPart value)

rdar://13486998