mlugg 107b65ec5d Sema: explain why we tried to call an extern fn at comptime
I recently saw a user hit the "comptime call of extern function" error,
and get confused because they didn't know why the scope was `comptime`.
So, use `explainWhyBlockIsComptime` on this and related errors to add
all the relevant notes.

The added test case shows the motivating situation.
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