This is another prune of dead code -- we never generate debug intrinsics
nowadays, therefore there's no need for these codepaths to run.

---------

Co-authored-by: Nikita Popov <github@npopov.com>

This flag was used to let us incrementally introduce debug records
into LLVM, however everything is now using records. It serves no
purpose now, so delete it.

These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

createCast in MergeFunctions did not consider ArrayTypes, which results
in the creation of a bitcast between ArrayTypes in the thunk function,
leading to an assertion failure in the provided test case.

The version of createCast in GlobalMergeFunctions does handle
ArrayTypes, so this common code has been factored out into the
IRBuilder.

DenseSet, SmallPtrSet, SmallSet, SetVector, and StringSet recently
gained C++23-style insert_range.  This patch replaces:

  Dest.insert(Src.begin(), Src.end());

with:

  Dest.insert_range(Src);

This patch does not touch custom begin like succ_begin for now.

MergeFunc uses the original function F as Thunk and creates a new function NewF for the original function F. Preserve F's comdat info on NewF instead of the thunk.

This fixes a crash when trying to lower comdat on the thunk during codegen.

Update writeThunkOrAlias to only create an alias or thunk if it is
actually needed. Drop discardable linkone_odr functions if they are not
used before.

PR: https://github.com/llvm/llvm-project/pull/128865

Avoid creating new calls to linkonce_odr/weak_odr functions when
merging 2 functions, as this may introduce an infinite call
cycle.

Consider 2 functions below, both present in 2 modules. 

Module X

--
define linkonce_odr void @"A"() {
  call void @"foo"()
}

define linkonce_odr void @"B"() {
  call void @"foo"()
}
--- 

Module Y
---
global @"g" = @"B"

define linkonce_odr void @"A"() {
  %l = load @"g"
  call void %l()
}

define linkonce_odr void @"B"() {
  call void @"foo"()
}
---

 @"A" and @"B" in both modules are semantically equivalent

Module X after function merging:

---
define linkonce_odr void @"A"() {
  call void @"foo"()
}

define linkonce_odr void @"B"() {
  call void @"A"()
}
---

Module Y is unchanged.

Then the linker picks @"A" from module Y and @"B" from module X. Now there's an infinite call cycle


PR: https://github.com/llvm/llvm-project/pull/125050

This modification will enable the usage of `MergeFunctions` as a
standalone library. Currently, `MergeFunctions` can only be applied to
an entire module. By adopting this change, developers will gain the
flexibility to reuse the `MergeFunctions` code within their own
projects, choosing which functions to merge; hence, promoting code
reusability. Notice that this modification will not break backward
compatibility, because `MergeFunctions` will still work as a pass after
the modification.

Identified with misc-include-cleaner.