The method `ts.CompilerHost.directoryExists` is optional, and was not
previously handled by our ts.CompilerHost wrapper for factory and
summary shims (GeneratedShimsHostWrapper).
TypeScript checks for the existence of this method and silently ignores
things like typeRoots if it's not found. This commit adds proper handling
of directoryExists() to the shim.
A test is also added which verifies typeRoots behavior works when shims
are enabled.
PR Close#27470
0d8ab323a7