Specifically the bad pattern was:
```
for (auto *vd : *caseStmt->getCaseBodyVariables()) { ... }
```
The problem is that the optional is not lifetime extended over the for loop. To
work around this, I changed the API of CaseStmt's getCaseBodyVariable methods to
never return the inner Optional<MutableArrayRef<T>>. Now we have the following 3
methods (ignoring const differences):
1. CaseStmt::hasCaseBodyVariables().
2. CaseStmt::getCaseBodyVariables(). Asserts if the case body variable array was
never specified.
3. CaseStmt::getCaseBodyVariablesOrEmptyArray(). Returns either the case body
variables array or an empty array if we were never given any case body
variable array.
This should prevent anyone else in the future from hitting this type of bug.
radar://49609717
There are multiple ways in which the Clang importer can produce an
initializer, and we have existing name shadowing rules to decide on
the best. Extend those rules to cover the case where a
Clang-importer-synthesized initializer collides with a C function
imported as an initializer. There's technically no reason to do the
latter because the former already exists, but some frameworks
currently depend on this.
Prior to this, the constraint solver was preferring the synthesized
initializer already (which is the right thing to do), for the wrong
reasons.
Address an annoying source compatibility issue with the introduction
of Data.withUnsafeBytes, which is ambiguous with Swift NIO's
implementatio of the same function. Do so with a narrow hackish name
shadowing rule (the Swift NIO version shadows the Foundation version)
that we will eventually generalize to something sensible.
Fixes rdar://problem/46850346.
Narrow the recently-introduced shadowing rule for types so that it only
applies to non-member types. Member types lack a reasonable syntax for
specifying precisely which module to look into, and there are a few use
cases where the type checker will pick a type that would be shadowed by
the new rule.
Fixes a source-compatibility regression introduced by the shadowing rule.
