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

[Reid Kleckner via cfe-commits]

rnk wrote:

We have internal reports of incorrect mangling substitutions after updating to 537b6541e8067d7ef7aa38791989fca6303b7fd from 799e9053641a6478d3144866a97737b37b87c260, and this change seems like the likely culprit because it's touching the Itanium mangling code. We'll see if that can be confirmed.

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