Report const declarations with values not known at compile time.

[pzipper]

The following program causes the compiler to crash:

const MyNumber = 42;

const Main = func(): i32 {
    // MyNumber();
};

const MyValue = Main();
//              ^^^^^^ not known at compile time

(functions cannot currently be called at compile time)

Instead, this should report a nice error message stating that the value is not known at compile time.