[clang] [Clang] Use correct evaluation contexts when instantiating a var without initializer (PR #140699)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

It would also be nice to get a reduction for: https://github.com/llvm/llvm-project/issues/140632 and see if we can add a test for that if it looks significantly different from the other cases.

https://github.com/llvm/llvm-project/pull/140699