[cmake] Disable GCC 9's -Winit-list-lifetime warning in ArrayRef

Summary:
This is a new warning which fires when one stores a reference to the
initializer_list contents in a way which may outlive the
initializer_list which it came from. In llvm this warning is triggered
whenever someone uses the initializer_list ArrayRef constructor.

This is indeed a dangerous thing to do (I myself was bitten by that at
least once), but it is not more dangerous than calling other ArrayRef
constructors with temporary objects -- something which we are used to
and have accepted as a tradeoff for ArrayRef's efficiency.

Currently, this warnings generates so much output that it completely
obscures any actionable warnings, so this patch disables it.

Reviewers: rnk, aaron.ballman

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70122