As part of the "RemoveDIs" project, BasicBlock::iterator now carries a
debug-info bit that's needed when getFirstNonPHI and similar feed into
instruction insertion positions. Call-sites where that's necessary were
updated a year ago; but to ensure some type safety however, we'd like to
have all calls to moveBefore use iterators.

This patch adds a (guaranteed dereferenceable) iterator-taking
moveBefore, and changes a bunch of call-sites where it's obviously safe
to change to use it by just calling getIterator() on an instruction
pointer. A follow-up patch will contain less-obviously-safe changes.

We'll eventually deprecate and remove the instruction-pointer
insertBefore, but not before adding concise documentation of what
considerations are needed (very few).

This PR replaces all `UndefValue` act as placeholders with `PoisonValue`
in `llvm/unittests`.

Rename the function to reflect its correct behavior and to be consistent
with `Module::getOrInsertFunction`. This is also in preparation of
adding a new `Intrinsic::getDeclaration` that will have behavior similar
to `Module::getFunction` (i.e, just lookup, no creation).

These are the final few places in LLVM that use instruction pointers to
insert instructions -- use iterators instead, which is needed for
debug-info correctness in the future. Most of this is a gentle
scattering of getIterator calls or not deref-then-addrofing iterators.
libfuzzer does require a storage change to keep built instruction
positions in a container though. The unit-test changes are very
straightforwards.

This leaves us in a position where libfuzzer can't fuzz on either of
debug-info records, however I don't believe that fuzzing of debug-info
is in scope for the library.

Reverts the above commit, as it updates a common header function and
did not update all callsites:

  https://lab.llvm.org/buildbot/#/builders/29/builds/382

This reverts commit 6481dc57612671ebe77fe9c34214fba94e1b3b27.

Uses the new InsertPosition class (added in #94226) to simplify some of
the IRBuilder interface, and removes the need to pass a BasicBlock
alongside a BasicBlock::iterator, using the fact that we can now get the
parent basic block from the iterator even if it points to the sentinel.
This patch removes the BasicBlock argument from each constructor or call
to setInsertPoint.

This has no functional effect, but later on as we look to remove the
`Instruction *InsertBefore` argument from instruction-creation
(discussed
[here](https://discourse.llvm.org/t/psa-instruction-constructors-changing-to-iterator-only-insertion/77845)),
this will simplify the process by allowing us to deprecate the
InsertPosition constructor directly and catch all the cases where we use
instructions rather than iterators.

Since we no longer support typed LLVM IR pointer types, the code can
be simplified into for example using PointerType::get directly instead
of using Type::getInt8PtrTy and Type::getInt32PtrTy etc.

Differential Revision: https://reviews.llvm.org/D156733

Value::setNameImpl() is used both to set and reset name of the value.
In destructor of Function all arguments get reset their names
(see Function::clearArguments()). If the arguments had their names set (e.g.
when the function was created with LLVMContex::DiscardValueNames == true)
then their ValueName entries referred by the function's symbol table must be
destructed. They are not destructed if LLVMContex::DiscardValueNames is set to
false because of the fast path in Value::setNameImpl(). See the new test cases
that demonstrate the problem. Without the fix they both crash in the function's
destructor.

In Value::setNameImpl() this patch narrows down the fast path return for
DiscardValueNames == true to allow destruction of ValueName entries if any.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D143487

Part of `dbg.addr` removal
Discussed in https://discourse.llvm.org/t/what-is-the-status-of-dbg-addr/62898

Differential Revision: https://reviews.llvm.org/D144801

Currently the only way to do this is to work with the instruction list directly.
This is part of a series of cleanup patches towards making BasicBlock::getInstList() private.

Differential Revision: https://reviews.llvm.org/D139142