Move the logic from `FailureDiagnostic::resolveType` into
`Solution::simplifyType` to allow completion to use it too. While
here, also handle cases where the placeholder is from a different
member of the equivalence class to the generic parameter.
In preparation for removing UnresolvedType in favor of ErrorType,
start printing ErrorType as `_` unless we've enabled debug printing.
`<<error type>>` should never be presented to the user, instead `_`
now just consistently means "unknown type".
`CodeCompletioString::getName()` was used only as the sorting keys in
`CodeCompletionContext::sortCompletionResults()` which is effectively
deprecated. There's no reason to check them in `swift-ide-test`. Instead,
check `printCodeCompletionResultFilterName()` that is actually used for
filtering.
This fixes code completion hitting llvm_unreachable("non-canonical or
unchecked type") in subst on a type containing an UnresolvedType.
Resolves rdar://problem/56726715
