qdoc: Implements the -no-link-errors option

qdoc: Implements the -no-link-errors option

The -no-link-errors flag tells qdoc not to print any link error
messages at all. This is useful for finding and fixing all non-link
errors in a module.

Task number: QTBUG-26870

Change-Id: Id4b0eebb6c0509c57d2f01763b6dedbfb6756a91
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>