[clangd] [modules] clangd uses stale PCM after prerequisite module construction fails

When processing a consumer with `--experimental-modules-support`, clangd attempts to build its own PCM for each imported module.

If the module interface listed in the compilation database no longer exists, this step fails. However, clangd then continues processing the consumer using the existing PCM specified by the consumer’s original -fmodule-file argument.

This fallback is not reported to the user, even though the existing PCM may no longer represent the current project state.

1. Build a named module and consumer.
2. Delete the module interface, leaving the CDB and PCM unchanged.
3. Open the consumer in clangd.

Logs:
```
Scanning modules dependencies for M.cppm failed: no such file
Failed to build module M
```
However, it retains -fmodule-file=M=.../M.pcm, loads that PCM, and publishes no diagnostic.

Expected: Report the failed module construction and potentially stale PCM fallback.
Tested with clangd 22.1.8 and --experimental-modules-support.

Reproduce the issue with clangd using --experimental-modules-support: build a named module and consumer, delete the module interface while leaving the compilation database and PCM unchanged, then open the consumer. Trace the failed module construction and the existing -fmodule-file fallback. Done means clangd reports the failed construction and potentially stale PCM use to the user.

Written by the indexing model from the issue text.