This looks sketchy, but I manually verified that the assertion should hold in all cases we currently have; it’s just hard to encode it into the types. The only times `host.getCurrentDirectory` is `undefined` are

1. The command line parser and typings installer each call the module name resolver in an ad-hoc way under certain circumstances, but they pass a hard-coded `compilerOptions` that lack `paths`, so this code path is never taken.
2. `Project` calls `getAutomaticTypeDirectives` with a `DirectoryStructureHost`, but that function never uses `host.getCurrentDirectory()`.

Changing `ModuleResolutionHost['getCurrentDirectory']` to be non-optional makes these call sites a bit of a nuisance.