Incremental parsing issue fixes

[dbukki]

Fixes #735

In accordance with the diagnosis outlined in https://github.com/Ericsson/CodeCompass/issues/735#issuecomment-2078918431 , the following fixes have been made:

1. A new CppAstNodeMetricsFileView db view has been added to manage the AST node ID <-> File ID association (_astNodeIdCache) in CppMetricsParser::CppMetricsParser.
2. The .size() call in SourceManager::removeFile has been replaced by a new isSingletonResult helper function in dbutil.