[clang] [clang] Add CLANG_ENABLE_NEW_CONST_INTERP_BY_DEFAULT cmake option (PR #199396)

[Timm Baeder via cfe-commits]

tbaederr wrote:

I was a bit split on the naming here, the cmake option misses the "experimental" bit, which is quite imporant.

https://github.com/llvm/llvm-project/pull/199396