[analyzer] support a mode to only show relevant lines in HTML diagnostics

HTML diagnostics can be an overwhelming blob of pages of code.
This patch adds a checkbox which filters this list down to only the
lines *relevant* to the counterexample by e.g. skipping branches which
analyzer has assumed to be infeasible at a time.

The resulting amount of output is much smaller, and often fits on one
screen, and also provides a much more readable diagnostics.

Differential Revision: https://reviews.llvm.org/D41378

llvm-svn: 322612