[Serialization] Read the initializer for interesting static variables before consuming it (#92353)

Close https://github.com/llvm/llvm-project/issues/91418

Since we load the variable's initializers lazily, it'd be problematic if
the initializers dependent on each other. So here we try to load the
initializers of static variables to make sure they are passed to code
generator by order. If we read any thing interesting, we would consume
that before emitting the current declaration.