build.d: Get path to vswhere

[JinShil]

This gets the path to vswhere: https://github.com/Microsoft/vswhere

vswhere is a single executable for locating Visual Studio installations and components. It's installed automatically by Visual Studio 2017 v15.2+ to a standard location at %ProgramFiles(x86)%\Microsoft Visual Studio\Installer\vswhere.exe. It is also downloadable from the GitHub repository linked above.

I intend to use it at

dmd/src/vcbuild/msvc-dmc.d

Lines 17 to 21 in 3d289d3

	enum defaultVSVersion = "10.0";
	auto cl = environment.get("MSVC_CC",
	environment.get("VCINSTALLDIR",
	`\Program Files (x86)\Microsoft Visual Studio %s\VC\`.format(defaultVSVersion))
	.buildPath("bin", "amd64", "cl.exe"));

and

dmd/src/vcbuild/msvc-lib.d

Lines 14 to 16 in 3d289d3

	auto lib = environment.get("MSVC_AR",
	environment.get("VCINSTALLDIR", `\Program Files (x86)\Microsoft Visual Studio 10.0\VC\`)
	.buildPath("bin", "amd64", "lib.exe"));

I think that code has a typo: notice that the path begins with \Program Files without a drive letter. Anyway, that code doesn't even work on my computer, so I can't get a build. I think vswhere will be a better solution for locating cl.exe and lib.exe.

msvc-dmc and msvc_lib will check the environment variable HOST_VSWHERE and execute it to locate cl.exe and lib.exe. build.d is responsible for setting HOST_VSWHERE, and that's what this PR does.

cc @rainers for your expertise

[dlang-bot]

Thanks for your pull request, @JinShil!

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + dmd#8596"

[wilzbach]

Better wrap this in a function as we can reuse it for a cross-platform download of the bootstrap compiler too, then.

[JinShil]

Done. I didn't change the existing bootstrap compiler download to avoid introducing issues. I can always do that as a followup.

[jacob-carlborg]

If the functions are chained standardPath could be made const.

[jacob-carlborg]

There's an extra space before Consider.