[msbuild] Improve how we handle the path to Xcode.

[rolfbjarne]

WIP WIP WIP

Mixed TODO/DONE list:

• Add support for an MSBuild property to set the Xcode location.
• Deprecate the file-based locations.
• Always set DEVELOPER_DIR when executing subprocesses.
• Don't set XCODE_DEVELOPER_USR_PATH anymore, xcrun should take care of this if needed.
• Detect Xcode as a symlink and warn/fail.
• Don't use static state, it plays badly with .NET's msbuild server

Fixes #11172.
Fixes #21762.

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[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

NET (empty diffs)

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

ℹ️ Generator diff

Generator Diff: vsdrops (html) vsdrops (raw diff) gist (raw diff) - Please review changes)

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[vs-mobiletools-engineering-service2]

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