Remove a bunch of on-top patches that are already part of llvm/clang 5.
Rebase the remaining ones and add backported ones.

Add also a README providing more information about the patches.

Task-number: QTCREATORBUG-18824
Task-number: QTCREATORBUG-17769
Change-Id: I9f6904b2e6d7c83de175e7b8c194e1a494cd2571
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>