[FileCheck] Fix --dump-input implicit pattern location

Currently, `--dump-input` implies that all `--implicit-check-not`
patterns appear on line 1 by printing annotations like:

```
       1: foo bar baz
not:1         !~~     error: no match expected
```

This patch changes that to:

```
          1: foo bar baz
not:imp1         !~~     error: no match expected
```

`imp1` indicates the first `--implicit-check-not` pattern.

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D77605