Prior to this fix, incremental rebuilds could fail to type check due to
missing ambient types from auto-discovered declaration files in @types
directories, or type roots in general. This was caused by the
intermediary `ts.Program` that is created for template type checking,
for which a `ts.CompilerHost` was used which did not implement the
optional `directoryExists` methods. As a result, auto-discovery of types
would not be working correctly, and this would retain into the
`ts.Program` that would be created for an incremental rebuild.
This commit fixes the issue by forcing the custom `ts.CompilerHost` used
for type checking to properly delegate into the original
`ts.CompilerHost`, even for optional methods. This is accomplished using
a base class `DelegatingCompilerHost` which is typed in such a way that
newly introduced `ts.CompilerHost` methods must be accounted for.
Fixes#38979
PR Close#39011
e9a8f9f705