Fix file-scope contract call handling in Solana

[Embers-of-the-Fire]

Should closes #1869

• fix a Solana sema panic when a state-variable initializer contains an external self-call and context.function_no is None
• limit the Solana accounts-required check to real function contexts so file-scope initializers no longer hit an unwrap or emit a spurious diagnostic
• add a regression test that asserts the initializer is preserved as an ExternalFunctionCall for uint public x = this.foo();