Sema: implement missing stage1 errors

test/cases/compile_errors/extern_function_pointer_mismatch.zig

@@ -0,0 +1,13 @@
+const fns = [_](fn(i32)i32) { a, b, c };
+pub fn a(x: i32) i32 {return x + 0;}
+pub fn b(x: i32) i32 {return x + 1;}
+export fn c(x: i32) i32 {return x + 2;}
+
+export fn entry() usize { return @sizeOf(@TypeOf(fns)); }
+
+// error
+// backend=stage2
+// target=native
+//
+// :1:37: error: expected type 'fn(i32) i32', found 'fn(i32) callconv(.C) i32'
+// :1:37: note: calling convention 'C' cannot cast into calling convention 'Unspecified'