[FileCheck] Report captured variables

[FileCheck] Report captured variables

Report captured variables in input dumps and traces.  For example:

```
$ cat check
CHECK: hello [[WHAT:[a-z]+]]
CHECK: goodbye [[WHAT]]

$ FileCheck -dump-input=always -vv check < input |& tail -8
<<<<<<
           1: hello world
check:1'0     ^~~~~~~~~~~
check:1'1           ^~~~~ captured var "WHAT"
           2: goodbye world
check:2'0     ^~~~~~~~~~~~~
check:2'1                   with "WHAT" equal to "world"
>>>>>>

$ FileCheck -dump-input=never -vv check < input
check2:1:8: remark: CHECK: expected string found in input
CHECK: hello [[WHAT:[a-z]+]]
       ^
<stdin>:1:1: note: found here
hello world
^~~~~~~~~~~
<stdin>:1:7: note: captured var "WHAT"
hello world
      ^~~~~
check2:2:8: remark: CHECK: expected string found in input
CHECK: goodbye [[WHAT]]
       ^
<stdin>:2:1: note: found here
goodbye world
^~~~~~~~~~~~~
<stdin>:2:1: note: with "WHAT" equal to "world"
goodbye world
^
```

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D83651