[clang] [clang] Implement instantiation context note for checking template parameters (PR #126088)

[Erich Keane via cfe-commits]

erichkeane wrote:

> Ping. I'll merge tomorrow if there are no further objections.

SGTM.  I haven't been paying SUPER close attention, but have been keeping an eye on this at least a little, and have no objections.

https://github.com/llvm/llvm-project/pull/126088