Use `MapVector` so `GCStrategyMap` can support forward and reverse
iterator, which is required in `AsmPrinter`.

Follow up to https://github.com/llvm/llvm-project/issues/123569

Identified with misc-include-cleaner.

It is simple to create the struct body up front, now that we have
transitioned to opaque pointers.

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.

IIUC the new pass system was designed with parallelism. This pass needs
to add some global variables into the current module, this is not
allowed by

[WritingAnLLVMPass](https://llvm.org/docs/WritingAnLLVMPass.html#the-functionpass-class),
so convert it to module pass, see FIXME in `GetFrameMap`.

Therefore, this will trigger assertion in `CodeGenPassBuilder`:
https://github.com/llvm/llvm-project/blob/effd47ed45e3badd756103346a7c3b9e1e939e5e/llvm/include/llvm/CodeGen/CodeGenPassBuilder.h#L200-L207
Will fix it in future.

Work towards removing method Type::getPointerTo.
Opaque ptr cleanup effort.