Revert "[analyzer] Model casts to bool differently from other numbers."

This seems to be causing quite a slowdown on our internal analyzer bot,
and I'm not sure why. Needs further investigation.

This reverts r180638 / 9e161ea981f22ae017b6af09d660bfc3ddf16a09.
[analyzer] scan-view: don't ever serve absolute paths.

At one point in time scan-view allowed absolute paths to reference files
within the server root, but this doesn't seem to be used anymore, and
caused problems if a server-root-relative path actually matched an
absolute path to an existing file. This patch just treats paths as
server-root-relative all the time.

PR15843