Fix race when copying lazy computed values for updateProgram

[sheetalkamat]

Fixes #2275

[Copilot]

Pull request overview

Fixes a reported data race (#2275) in the TypeScript-go compiler’s Program cloning path (updateProgram/UpdateProgram) by making certain lazily computed caches safe to copy across program updates.

Changes:

• Introduced a generic lazyValue[T] helper to safely manage lazily computed cached values with a “try-get-if-initialized” path.
• Updated Program to store unresolvedImports and knownSymlinks using lazyValue, and adjusted GetUnresolvedImports/GetSymlinkCache accordingly.
• Modified UpdateProgram to copy only fully-initialized cached values for these fields, avoiding concurrent reads of partially initialized state.