CppTools: Do not block GUI thread for completions

The GUI was blocked while waiting for the parsed document. Now the
blocking operation is executed in the completion thread.

Task-number: QTCREATORBUG-11037
Task-number: QTCREATORBUG-11433
Change-Id: Ia7c1b1b7eea0ba75010ff667ba05273c62c18491
Reviewed-by: hjk <hjk121@nokiamail.com>