All deserialized VarDecl initializers are EvaluatedStmt, but not all
EvaluatedStmt initializers are from a PCH. Calling
`VarDecl::hasInitWithSideEffects` can trigger constant evaluation, but
it's hard to know ahead of time whether that will trigger
deserialization - even if the initializer is fully deserialized, it may
contain a call to a constructor whose body is not deserialized. By
caching the result of `VarDecl::hasInitWithSideEffects` and populating
that cache during deserialization we can guarantee that calling it won't
trigger deserialization regardless of the state of the initializer.
This also reduces memory usage by removing the `InitSideEffectVars` set
in `ASTReader`.
rdar://154717930
