Sema: emit error when pointer to extern function is called

test/cases/compile_errors/invalid_extern_function_call.zig

@@ -0,0 +1,16 @@
+const x = @extern(*const fn() callconv(.C) void, .{ .name = "foo" });
+
+pub fn entry0() void {
+    comptime x();
+}
+
+pub fn entry1() void {
+    @call(.always_inline, x, .{});
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :4:15: error: comptime call of extern function pointer
+// :8:5: error: inline call of extern function pointer