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

[via llvm-commits]

cmtice wrote:

I've updated this to address most review comments; I have not added the md5 checksum yet; will get to that soon.

I updated the container to remove many of the downloaded files from clang; I kept the entire include directory, and the entire lib directory. In the bin directory I kept clang-cl.exe and lld-link.exe.

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 check 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.

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