[dataflow] add HTML logger: browse code/cfg/analysis timeline/state

With -dataflow-log=/dir we will write /dir/0.html etc for each
function analyzed.

These files show the function's code and CFG, and the path through
the CFG taken by the analysis. At each analysis point we can see the
lattice state.

Currently the lattice state dump is not terribly useful but we can
improve this: showing values associated with the current Expr,
simplifying flow condition, highlighting changes etc.

(Trying not to let this patch scope-creep too much, so I ripped out the
half-finished features)

Demo: https://htmlpreview.github.io/?https://gist.githubusercontent.com/sam-mccall/1746985bf13406bd19181af281aea9ff/raw/9718fdd48406dabccb3092acd983b4bd55da9dfa/analysis.html

Differential Revision: https://reviews.llvm.org/D146591