Add msvc-14.0 and clang-win to GHA Windows jobs

[Flamefire]

The clang-win job requires embed-manifest-via=linker

See run at https://github.com/boostorg/boost-ci/runs/6789632405?check_suite_focus=true
and upstream issue bfgroup/b2#159

@pdimov We had that in Boost.Unordered. I think it makes sense to put that here instead of unconditionally add it everywhere.

Note that the Appveyor clang-win job works without this flag: https://ci.appveyor.com/project/Flamefire/boost-ci/builds/43769849/job/4a9kgqgbb9dmiek0

Hence I only add it for GHA.

[pdimov]

I can't think of anything wrong with using the linker unconditionally. I've suggested this be made default in b2 but the suggestion hasn't been taken up.

Appveyor probably works because it uses an earlier Windows SDK, so the clang-win toolset is able of autodetecting the path to mt.exe there.

[Flamefire]

I can't think of anything wrong with using the linker unconditionally. I've suggested this be made default in b2 but the suggestion hasn't been taken up.

If that gets implemented we can remove the workaround here. Until then I'd try and stay as close to what a user would do, which is likely using b2 without that flag. Hence only use that flag where we know it to be required.

[codecov]

Codecov Report

Merging #169 (0c22052) into master (376479c) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #169   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           20        20           
  Branches        10        10           
=========================================
  Hits            20        20           

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