[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!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191909 91177308-0d34-0410-b5e6-96231b3b80d8
[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!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191910 91177308-0d34-0410-b5e6-96231b3b80d8