AnalyzerBase: Fix opening links with Windows drive letters

Change-Id: Iba365f9abeff2573296295e50a6100cc480374b4
Reviewed-by: hjk <hjk121@nokiamail.com>
CppEditor: Merge AddIncludeForForwardDeclaration into AddIncludeForUndefinedIdentifier

* Fixes multiple addition of same include.
* Takes over the check of AddIncludeForForwardDeclaration for forward
  headers.

Task-number: QTCREATORBUG-9704
Change-Id: I84629d35ae433385942a9157e5d32ef04159d07f
Reviewed-by: Christian Stenger <christian.stenger@digia.com>