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
bce8fced41