runtime: use std::max_align_t over max_align_t (NFC)

[compnerd]

MSVC does not provide a full C99 environment. Use the C++ equivalent
spelling for the value. This allows the build to succeed on Windows.

Replace this paragraph with a description of your changes and rationale. Provide links to external references/discussions if appropriate.

Resolves SR-NNNN.

[compnerd]

@swift-ci please test and merge

[xedin]

@compnerd Looks like this broke Ubuntu 14.04 bot - https://ci.swift.org/job/oss-swift-incremental-RA-linux-ubuntu-14_04/4486/

[compnerd]

Weird, std::max_align_t is part of C++11, it should be present. http://en.cppreference.com/w/cpp/types/max_align_t indicates that the included header should define it. Perhaps there is another header that can be included as a workaround.

[millenomi]

@compnerd I think you need to #include <cstddef> instead of including the C header #include "stddef.h".

[millenomi]

Can we revert, or correct this with a new PR that is tested on 14.04?

[xedin]

Maybe #17101 is going to fix it?

[compnerd]

@millenomi - trying to setup a 14.04 environment to look into this. We should certainly look into correcting this :-)

The interesting thing is that the change does include cstddef. Perhaps @xedin's patch will fix it by removing the inclusion of the C stddef.h include.

[xedin]

Looks like my changes didn't help but it's good to have them anyway...

[jrose-apple]

There's a stddef.h in include/swift/Runtime/Portability.h too. Is that (transitively) included?

[xedin]

Maybe, I've opened #17126 to deal with that as well...

[compnerd]

I'm still trying to build in a VM :-(

[rudkx]

#17126 does not appear to have fixed this, so I've opened #17139 to revert the original change until this can be sorted out.

[compnerd]

I didn't expect building against 14.04 to be so challenging. Unfortunately, this change is needed to build for Windows, so both states are less than ideal. The solution seems to be that we need to special case the 14.04 build. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56019 indicates that GCC 4.8 has a bug in the C++11 standard library implementation and is missing the definition of std::max_align_t. This has been fixed in libstdc++ which is distributed as part of GCC 4.9. I'll upload a workaround for this.

[rudkx]

I'm happy to hold off on merging the revert if you can land something relatively quickly that resolves the issue, but we shouldn't let this keep the CI red for much longer.

[compnerd]

@rudkx, @xedin I think that #17140 should repair the 14.04 build.