[DebugInfo][RemoveDIs] Turn on non-instrinsic debug-info by default

This patch causes all variable-location debug-info to be converted into
non-intrinsic records as they passes through the optimisation /
instrumentation passes. There's a brief introduction here [0] and a more
detailed thread on what this means on discourse at [1].

If this commit is breaking your downstream tests, please see comment 12 in
[1], which documents the kind of variation in tests we'd expect to see from
this change and what to do about it.

[0] https://llvm.org/docs/RemoveDIsDebugInfo.html
[1] https://discourse.llvm.org/t/rfc-instruction-api-changes-needed-to-eliminate-debug-intrinsics-from-ir/68939

Author: jmorse

llvm/lib/IR/BasicBlock.cpp

@@ -34,7 +34,7 @@ cl::opt<bool>
     UseNewDbgInfoFormat("experimental-debuginfo-iterators",
                         cl::desc("Enable communicating debuginfo positions "
                                  "through iterators, eliminating intrinsics"),
-                        cl::init(false));
+                        cl::init(true));
 
 DPMarker *BasicBlock::createMarker(Instruction *I) {
   assert(IsNewDbgInfoFormat &&