[clang] Reland "[Clang] Enable -Wunused-template under -Wall" (PR #208001)

[Aditya Medhane via cfe-commits]

flash1729 wrote:

thanks for flagging @Thyre, this breakage is similar to past occurrences, I am working on a fix and will ship the patch soon

https://github.com/llvm/llvm-project/pull/208001