Update Sema::DiagRuntimeBehavior() to take an optional Stmt* to indicate the code...

Update Sema::DiagRuntimeBehavior() to take an optional Stmt* to indicate the code the diagnostic is associated with.

This Stmt* is unused, but we will use it shortly for pruning diagnostics associated
with unreachable code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@126286 91177308-0d34-0410-b5e6-96231b3b80d8