[FileCheck] Implement -dump-input-context

This patch is motivated by discussions at each of:

* <https://reviews.llvm.org/D81422>
* <http://lists.llvm.org/pipermail/llvm-dev/2020-June/142369.html>

When input is dumped as specified by `-dump-input=fail`, this patch
filters the dump to show only input lines that are the starting lines
of error diagnostics plus the number of contextual lines specified
`-dump-input-context` (defaults to 5).

When `-dump-input=always`, there might be not be any errors, so all
input lines are printed, as without this patch.

Here's some sample output with `-dump-input-context=3 -vv`:

```
<<<<<<
           .
           .
           .
          13: foo
          14: foo
          15: hello world
check:1       ^~~~~~~~~~~
          16: foo
check:2'0     X~~ error: no match found
          17: foo
check:2'0     ~~~
          18: foo
check:2'0     ~~~
          19: foo
check:2'0     ~~~
           .
           .
           .
          27: foo
check:2'0     ~~~
          28: foo
check:2'0     ~~~
          29: foo
check:2'0     ~~~
          30: goodbye word
check:2'0     ~~~~~~~~~~~~
check:2'1     ?            possible intended match
          31: foo
check:2'0     ~~~
          32: foo
check:2'0     ~~~
          33: foo
check:2'0     ~~~
           .
           .
           .
>>>>>>
```

Reviewed By: mehdi_amini, arsenm, jhenderson, rsmith, SjoerdMeijer, Meinersbur, lattner

Differential Revision: https://reviews.llvm.org/D82203