swift234

[Coverage] Fix crash when assigning counter to orphan if_expr

When visited by an ASTWalker, an if_expr nested within a
patter_binding_decl has no Parent. This leads to a crash while assigning
counters for the if_expr's "else" branch: there is no enclosing region,
so grabbing the current region is impossible.

We can handle this case safely by using a new leaf counter for the
"else" branch.

Swift PR-1116, rdar://problem/23256795

(cherry picked from commit f7029d901c8f4a2c5ef46efe9cb9e1e21f1174a8)