Add test case for crash triggered in `?`.
Current number of unresolved compiler crashers: 79 (5179 resolved)
Assertion failure in [`lib/AST/Type.cpp (line 1280)`](https://github.com/apple/swift/blob/master/lib/AST/Type.cpp#L1280):
```
Assertion `Result && "Case not implemented!"' failed.
When executing: swift::CanType swift::TypeBase::getCanonicalType()
```
Assertion context:
```
}
}
// Cache the canonical type for future queries.
assert(Result && "Case not implemented!");
CanonicalType = Result;
return CanType(Result);
}
```
Stack trace:
```
swift: /path/to/swift/lib/AST/Type.cpp:1280: swift::CanType swift::TypeBase::getCanonicalType(): Assertion `Result && "Case not implemented!"' failed.
Stack dump:
0. Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28447-result-case-not-implemented-failed.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28447-result-case-not-implemented-failed-778155.o
<unknown>:0: error: unable to execute command: Aborted
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```
