RFC: Download and build Polly when USE_POLLY=1

[MatthiasJReisinger]

This enables the build process to automatically download and build Polly. An earlier version of this functionality has already been part of #16531, but as proposed by @tkelman I've set up this separate PR for it.

In its current state, this PR allows to enable Polly more easily - it only requires the following lines in a Make.user file and its no longer needed to download/build Polly+LLVM yourself:

USE_POLLY:=1
LLVM_VER:=svn

As you can see, it is currently required to use the svn version of LLVM for this. However, if desired I can generalize this and test whether it is possible to use older versions of Polly/LLVM as well here (so far this was not my primary focus for this).

If there's anything that's unclear or if there's something important that I missed, I'll be glad about questions/comments/advice.
@TobiG @vtjnash @timholy

[tobiasgrosser]

From my perspective this looks good. I also don't think it makes any sense to support older version of Polly. They are unlikely to be impactful as of today and we will not port your changes to these older versions, such that they will continue to only work occasionally. For users it is a lot easier to just not support Polly, rather then having them to figure out that it just does not work as expected.

[Keno]

This looks fine to me. I'm also fine with only supporting it on svn while we are experimenting with it.

[tkelman]

LGTM as-is. As long as using Polly with Julia is an experimental disabled-by-default option it doesn't need to be distribution-worthy. Just curious, how much are you needing to change upstream to make this all work?

[MatthiasJReisinger]

Thank you for the feedback so far. For now the plain process of making Polly available in Julia should be done with this. Currently I am experimenting to identify problems that Polly may have with the LLVM code that Julia emits. So besides adapting Polly itself there might also follow changes to Julia's codegen, this however is not yet really predictable.

[MatthiasJReisinger]

I apologize, this didn't answer your question @tkelman. For the next changes it will probably not be necessary to have them upstream. For #16531 and this here though I was a bit enthusiastic to get them upstream, so that others can start using Polly :)

[tkelman]

When I say "upstream" I generally mean changes to llvm/polly, not your PR's here.

[MatthiasJReisinger]

Okay, this was a misunderstanding then, thank you for the clarification.