The diagnosticc engine is keeping track of state which might modify parsing/typechecking behaviour. In the added test case the `fatalErrorOccurred` flag was during the first completion. The flag was still `true` for the second completion, causing parsing/typechecking to behave slightly differently. Because of this, the ExprRewriter failed when applying a constraint system solution, not properly cleaning up its `ExprStack`.
This PR tackles both issues:
1) Reset the `hadError` flags in the diagnostics engine
2) Clean up the `ExprRewriter`’s `ExprStack` when rewriting a target fails.
Either of these changes fixes the crash in the test case but I think both could manifest through different code paths in different scenarios.
Fixes rdar://76051976 [SR-14430]
