[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

llvm/include/llvm/ADT/ArrayRef.h

@@ -97,9 +97,19 @@ namespace llvm {
     /*implicit*/ constexpr ArrayRef(const T (&Arr)[N]) : Data(Arr), Length(N) {}
 
     /// Construct an ArrayRef from a std::initializer_list.
+#if LLVM_GNUC_PREREQ(9, 0, 0)
+// Disable gcc's warning in this constructor as it generates an enormous amount
+// of messages. Anyone using ArrayRef should already be aware of the fact that
+// it does not do lifetime extension.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Winit-list-lifetime"
+#endif
     /*implicit*/ ArrayRef(const std::initializer_list<T> &Vec)
     : Data(Vec.begin() == Vec.end() ? (T*)nullptr : Vec.begin()),
       Length(Vec.size()) {}
+#if LLVM_GNUC_PREREQ(9, 0, 0)
+#pragma GCC diagnostic pop
+#endif
 
     /// Construct an ArrayRef<const T*> from ArrayRef<T*>. This uses SFINAE to
     /// ensure that only ArrayRefs of pointers can be converted.