Research how to suspend/resume libclang to prevent file locks

When libclang does parsing/reparsing it may temporarily open header files and hold handlers to them if they were not passed into libclang call as unsaved files.

In case libclang does some complicated work the period of time when files might be locked may last several seconds. During that time it's possible that some headers are modified and saved but libclang does not know about it.

The idea is to research how we can force libclang stop processing files (or totally suspend) for some period of time and then to resume it's work.

This will help to deal with problems similar to QTCREATORBUG-15449