Add test case for crash triggered in `?`.
Current number of unresolved compiler crashers: 87 (5180 resolved)
Stack trace:
```
Stack dump:
0. Program arguments: /path/to/swift/bin/swift -frontend -c -primary-file validation-test/compiler_crashers/28456-unreachable-executed-at-swift-lib-ast-module-cpp-614.swift -target x86_64-unknown-linux-gnu -disable-objc-interop -module-name main -o /tmp/28456-unreachable-executed-at-swift-lib-ast-module-cpp-614-10867a.o
1. While type-checking 'a' at validation-test/compiler_crashers/28456-unreachable-executed-at-swift-lib-ast-module-cpp-614.swift:9:1
2. While resolving type e at [validation-test/compiler_crashers/28456-unreachable-executed-at-swift-lib-ast-module-cpp-614.swift:11:5 - line:11:5] RangeText="e"
<unknown>:0: error: unable to execute command: Aborted
<unknown>:0: error: compile command failed due to signal (use -v to see invocation)
```
