Now the main span focuses on the erroneous not-a-function callee,
while showing the entire call expression is relegated to a secondary
span. In the case where the erroneous callee is itself a call, we
point out the definition, and, if the call expression spans multiple
lines, tentatively suggest a semicolon (because we suspect that the
"outer" call is actually supposed to be a tuple).
The new `bug!` assertion is, in fact, safe (`confirm_builtin_call` is
only called by `check_call`, which is only called with a first arg of
kind `ExprKind::Call` in `check_expr_kind`).
Resolves#51055.
