LLVM toolset for VS 2019 does not exist unless VS 2017 is installed

[gregbedwell]

Bugzilla Link	41948
Version	unspecified
OS	Windows NT
CC	@zmodem,@jryans

Extended Description

Reported by a team here who are experimenting with building their codebase with clang-cl:

"""
The bug we observed is:

If you install the LLVM Compiler Toolchain Visual Studio extension for VS 2019 on a machine without VS 2017 (or without the VS 2017 extension), the LLVM platform toolset does not exist.

You can reproduce this by:

• Install the LLVM toolchain and VS 2019 extension on a machine without VS 2017, observe that the LLVM toolset does not appear as an option under Platform Toolset in the IDE.
• Install VS 2017, the LLVM toolset is still missing.
• Install the LLVM toolchain extension for VS 2017, the LLVM toolset now exists and works.
  """

[zmodem]

Sadly, there is nobody really maintaining the extension at this point.

The code is under llvm/tools/msbuild/ and patches are welcome.