[clang] [clang][import] Evaluate constexpr variable initialzers (PR #208794)

[Timm Baeder via cfe-commits]

tbaederr wrote:

I was not sure if this is the best place to do this, or if we maybe should do this regardless of the evaluator in use, since `importInitializer` does not import the `APValue`. But then again it could just copy the `APValue` with the current interpreter.

https://github.com/llvm/llvm-project/pull/208794