When we run the Preprocessor from the CppComprehensionEngine of
the language server, we don't want the preprocessor to crash if it
encounters an invalid preprocessor statement (for example, an #endif
statement without an accompanying previous #if statement).
To achieve this, this commit adds an "ignore_invalid_statements" flag
to the preprocessor which is set by the CppComprehensionEngine.
Fixes#11064.
c3c2fe153b