README.md: Should we mention using the VS Installer for LLVM?

[StephanTLavavej]

The test instructions say:

Have LLVM's bin directory on the PATH. Simply using LLVM's installer and choosing to add LLVM to your PATH during installation is the easiest way to get LLVM's bin directory on your PATH.

The VS Installer is capable of installing LLVM. (Although it isn't selected by default, an installation can easily be modified, without having to reboot.) Additionally, the STL is synced to this version - we don't increase the STL's minimum required version until the VS Installer has updated their version. (Although in the past, I believe we did so immediately, without waiting for a public Preview to ship - the idea being that STL changes would flow into VS at the same time. Now that STL changes are immediately available on GitHub, we'd need to wait for a Preview if we want to tell people to use it for testing.)

Should we modify the instructions to explain that "C++ Clang tools for Windows" can be selected in the VS Installer, and then locally added to the PATH via:

set PATH=C:\Program Files (x86)\Microsoft Visual Studio\2019\Preview\VC\Tools\Llvm\x64\bin;%PATH%

[StephanTLavavej]

This was fixed by #1153. We now recommend using the VS Installer to get LLVM, and the VS Command Prompts were enhanced to automatically add it to the PATH.