[DebugInfo] Soft-disable the production of debug intrinsics

This patch switches the --experimental-debuginfo-iterators flag to be
stored to an otherwise unused cl-opt. This is a deliberate attempt to break
downstream tests that are relying on the use of debug intrinsics, because
they're imminently going away! If this commit breaks your tests, please
just revert the commit upstream, and then make contact with us here:

https://discourse.llvm.org/t/psa-ir-output-changing-from-debug-intrinsics-to-debug-records/79578

So that we can work out whether there's any further transition work needed
to support the move away from using debug intrinsics.

Author: jmorse

llvm/lib/IR/BasicBlock.cpp

@@ -30,11 +30,28 @@ using namespace llvm;
 #define DEBUG_TYPE "ir"
 STATISTIC(NumInstrRenumberings, "Number of renumberings across all blocks");
 
+// This cl-opt exists to control whether variable-location information is
+// produced using intrinsics, or whether DbgRecords are produced. However,
+// it's imminently being phased out, so give it a flag-name that is very
+// unlikely to be used anywhere.
+//
+// If you find yourself needing to use this flag for any period longer than
+// five minutes, please revert the patch making this change, and make contact
+// in this discourse post, where we can discuss any further transition work
+// that might be needed to remove debug intrinsics.
+//
+// https://discourse.llvm.org/t/psa-ir-output-changing-from-debug-intrinsics-to-debug-records/79578
 cl::opt<bool> UseNewDbgInfoFormat(
-    "experimental-debuginfo-iterators",
-    cl::desc("Enable communicating debuginfo positions through iterators, "
-             "eliminating intrinsics. Has no effect if "
-             "--preserve-input-debuginfo-format=true."),
+    "dont-pass-this-flag-please-experimental-debuginfo", cl::Hidden,
+    cl::init(true));
+
+// This cl-opt collects the --experimental-debuginfo-iterators flag and then
+// does nothing with it (because the it gets stored into an otherwise unused
+// cl-opt), so that we can disable debug-intrinsic production without
+// immediately modifying lots of tests. If your tests break because of this
+// change, please see the next comment up.
+static cl::opt<bool> DeliberatelyUnseenDbgInfoFlag(
+    "experimental-debuginfo-iterators", cl::Hidden,
     cl::init(true));
 
 DbgMarker *BasicBlock::createMarker(Instruction *I) {