[analyzer] Add new debug helper clang_analyzer_warnIfReached.

This will emit a warning if a call to clang_analyzer_warnIfReached is
executed, printing REACHABLE. This is a more explicit way to declare
expected reachability than using clang_analyzer_eval or triggering
a bug (divide-by-zero or null dereference), and unlike the former will
work the same in inlined functions and top-level functions. Like the
other debug helpers, it is part of the debug.ExprInspection checker.

Patch by Jared Grubb!
[analyzer] Replace bug category magic strings with shared constants.

One small functionality change is to bring the sizeof-pointer checker in
line with the other checkers by making its category be "Logic error"
instead of just "Logic". There should be no other functionality changes.

Patch by Daniel Marjamäki!