[clang] [C++26][clang] Implement P2795R5 'Erroneous behaviour for uninitialized reads' (PR #177614)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Other than not having this as an extension, this patch looks about right to me.  I'd like someone else to take a look, but this otherwise seems acceptable.

https://github.com/llvm/llvm-project/pull/177614