[NFCI][RemoveDIs] Build LLVM with RemoveDIs iterators

This commit flips a bit to make LLVM build with "debuginfo iterators",
causing BasicBlock::iterator to contain a bit that's used for debug-info
purposes. More about this can be read on Discourse [0], but the runtime
impact of this should be negligable (iterators usually end up being
inlined), and there should be no change to LLVMs behaviour as a result of
this commit.

What this does mean though, is that roughly 400 debug-info tests where
we've added "--try-experimental-debuginfo-iterators" to RUNlines are going
to start operating in RemoveDIs mode. These are already tested on the
new-debug-iterators buildbot [1], and I've even tested with asan, so I'm
not _expecting_ any turbulence.

[0] https://discourse.llvm.org/t/rfc-instruction-api-changes-needed-to-eliminate-debug-intrinsics-from-ir/68939
[1] https://lab.llvm.org/buildbot/#/builders/275

Author: jmorse

llvm/CMakeLists.txt

@@ -654,7 +654,7 @@ option(LLVM_EXTERNALIZE_DEBUGINFO
   "Generate dSYM files and strip executables and libraries (Darwin Only)" OFF)
 
 option(LLVM_EXPERIMENTAL_DEBUGINFO_ITERATORS
-  "Add extra Booleans to ilist_iterators to communicate facts for debug-info" OFF)
+  "Add extra Booleans to ilist_iterators to communicate facts for debug-info" ON)
 
 set(LLVM_CODESIGNING_IDENTITY "" CACHE STRING
   "Sign executables and dylibs with the given identity or skip if empty (Darwin Only)")