Auto merge of #3740 - flip1995:const_assert_macro, r=oli-obk

Macro check for assertion_on_constants lint

The `assertion_on_constants` lint currently has following output for this code [Playground](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=6f2c9df6fc50baf847212d3b5136ee97):
```rust
macro_rules! assert_const {
    ($len:expr) => {
        assert!($len > 0);
    }
}

fn main() {
    assert_const!(3);
    assert_const!(-1);
}
```
```
warning: assert!(const: true) will be optimized out by the compiler
 --> src/main.rs:3:9
  |
3 |         assert!($len > 0);
  |         ^^^^^^^^^^^^^^^^^^
...
8 |     assert_const!(3);
  |     ---------------- in this macro invocation
  |
  = note: #[warn(clippy::assertions_on_constants)] on by default
  = help: remove it
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants

warning: assert!(const: false) should probably be replaced
 --> src/main.rs:3:9
  |
3 |         assert!($len > 0);
  |         ^^^^^^^^^^^^^^^^^^
...
9 |     assert_const!(-1);
  |     ----------------- in this macro invocation
  |
  = help: use panic!() or unreachable!()
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assertions_on_constants
```

This is contradictory. This lint should not trigger if the `assert!` is in a macro itself.

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