[clang][c23] Avoid assertion on an invalid static constexpr variable (#175927)
In C static variables should have constant expressions in initializers
so we were checking this twice for constexpr variables and failing with
an assertion that was makes sure we don't do it.
This patch postpones the check just like it is done for file
scope constexpr variables in C already.
Fixes https://github.com/llvm/llvm-project/issues/173605