Enhance Sema::DiagRuntimeBehavior() to delay some diagnostics to see if the related...

Enhance Sema::DiagRuntimeBehavior() to delay some diagnostics to see if the related code is reachable.  This suppresses some
diagnostics that occur in unreachable code (e.g., -Warray-bound).

We only pay the cost of doing the reachability analysis when we issue one of these diagnostics.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126290 91177308-0d34-0410-b5e6-96231b3b80d8