[Dexter] Allow Dexter watch commands to specify a range of acceptable FP values

This patch adds an optional argument to DexExpectWatchBase, float_range,
which defines a +- acceptance range for expected floating point values.
If passed, this assumes every expected value to be a floating point
value, and an exception will be thrown if this is not the case.

Differential Revision: https://reviews.llvm.org/D124511