RFC: Bump toolchain requirements

Hi,

the policy for updating the minimum compiler versions able to build llvm (and as a side-effect, the option of using features of a newer C++ standard) is here:

  https://llvm.org/docs/DeveloperPolicy.html#toolchain

The current compiler requirements are documented here:

  https://llvm.org/docs/GettingStarted.html#host-c-toolchain

I don't know if buildbots verify those versions.

I'm wondering if anyone has strong feelings for or against bumping the minimum compiler requirements to:

* GCC 7.4 (Dec 2018 - https://gcc.gnu.org/gcc-7)
* Clang 6.0 (Mar 2018 - https://lists.llvm.org/pipermail/llvm-announce/2018-March/000077.html)
* MSVC 2019
* AppleClang ??

Those versions of GCC and Clang are available on Ubuntu 18.04 LTS.

  https://packages.ubuntu.com/bionic-updates/clang
  https://packages.ubuntu.com/bionic-updates/gcc

Ubuntu 16.04 LTS is end-of-standard-support this month:

  https://wiki.ubuntu.com/Releases

Those compilers have much-improved C++17 support compared to the current baseline.

Thanks,

Stephen.

The policy for Visual Studio has been to support the two last major
releases and I don't see a reason to deviate from it. The next major
release will be Visual Studio 2022 [1].

Michael

[1] https://devblogs.microsoft.com/visualstudio/visual-studio-2022/

Can't say I have any strong feelings on this - if it's consistent with
the policy (is it?) & there's something to be gained by it (sounds
like it), great.

(might want to check this recent thread
https://lists.llvm.org/pipermail/cfe-dev/2021-April/068034.html in
case there's anything interesting there - and maybe summarize some/any
of the previous discussions to bump the minimum version and why any
counterarguments in those discussions are now addressed)