[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