[FlowSensitive] Log analysis progress for debugging purposes

The goal is to be able to understand how the analysis executes, and what its
incremental and final findings are, by enabling logging and reading the logs.
This should include both framework and analysis-specific information.

Ad-hoc printf-debugging doesn't seem sufficient for my understanding, at least.
Being able to check in logging, turn it on in a production binary, and quickly
find particular analysis steps within complex functions seem important.

This can be enabled programmatically through DataflowAnalysisOptions, or
via the flag -dataflow-log. (Works in unittests, clang-tidy, standalone
tools...)

Important missing pieces here:
 - a logger implementation that produces an interactive report (HTML file)
   which can be navigated via timeline/code/CFG.
   (I think the Logger interface is sufficient for this, but need to prototype).
 - display of the application-specific lattice
 - more useful display for the built-in environment
   (e.g. meaningful & consistent names for values, hiding redundant variables in
   the flow condition, hiding unreachable expressions)

Differential Revision: https://reviews.llvm.org/D144730