CppTools: Fix race conditions in CppEditorSupport

...when accessing m_lastSemanticInfo by introducing semanticInfo() and
setSemanticInfo(), which lock the appropriate mutex.

Task-number: QTCREATORBUG-11367
Change-Id: If8ac6b8e6d576dfd1869c98a7ff7952ec97f530e
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>