[clang][dataflow] Add initial sign analysis

This patch adds an initial implementation for sign analysis, with the
following lattice (T: top, N: negative, Z: zero, P: positive, B: bottom):
  T
/ | \
N Z P
\ | /
  B
The lattice is implemented with `BoolValue` properties attached to other
`Value`s.

Differential Revision: https://reviews.llvm.org/D136668