[clang] [clang] more useful error message for decomposition declaration missing initializer (PR #127924)

[Kevin Bravo via cfe-commits]

kbrav wrote:

> @kbrav Are you still working on this? @AaronBallman left a few comments

I am now, let me know if it got intercepted in the meantime.  It looks like it was briefly but then abandoned.

https://github.com/llvm/llvm-project/pull/127924