[llvm] [CI][Github] Install Clang in Windows container (PR #164519)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> The old container size was 17.7 GB. The new container size is 23.2 GB. This is not making a lot of sense to me -- I checked the size of the stuff I kept from the new clang download, and it's only a total of 1.2 GB (bin: 469MB; lib:665MB; include: 78MB). I entered the container and checked and the new stuff did indeed all get deleted, so I have no idea where the other new 4.3 GB came from.

You need to delete the files in the same layer (RUN step) that you download them in. Otherwise you'll end up with one layer downloading them and then another layer deleting them, and both layers get preserved due to how docker works.

https://github.com/llvm/llvm-project/pull/164519