[IRGenSIL][DebugInfo] Fix location of Coroutine/Async exit code
Exit code should not inherit the location of the previous basic block;
instead, it should contain line zero to denote that this is not
associated with any user code.
Without this patch, stepping over code code like:
```
1. yielding mutate {
2. yield &member_int
3. blah
4. blah
}
```
Would go from line 3, to line 4, to line 2.
rdar://149106144
