Parsing of dependent headers should be given lower priority and not block active file parsing #5128

Right now, the parsing of dependent headers will block all other tag parsing operations, even for high priority work such as newly opened files. The impact is most severe on 1st launch when none of the system headers have been parsed. This behavior can be observed via setting the C_Cpp.loggingLevel set to "Debug" and then opening a new C++ file or changing an open file and saving it -- if the logging shows header files are being tag parsed instead of the higher priority active file, then it's this bug (if other source files are getting parsed, then it's a different issue).

This issue also causes "Pause parsing" and "workspace parsing priority" to be bypassed (while the dependent headers are being parsed).

This is an old/known issue, but I couldn't locate an existing bug.