[clang] [clang-tools-extra] [libcxx] [clang] improved preservation of template keyword (PR #133610)

[Alexander Kornienko via cfe-commits]

alexfh wrote:

#134748 doesn't fix errors in the original code, unfortunately. Now we're getting `out-of-line definition of 'X' does not match any declaration in 'Y'`. It's hard to tell whether the original code is correct, but I suppose that the purpose of this change was not to make Clang stricter?

I'm re-running reduction with Clang after #134748

https://github.com/llvm/llvm-project/pull/133610