[FileCheck] Fix numeric error propagation

A more general name might be match-time error propagation.  That is,
it's conceivable we'll one day have non-numeric errors that require
the handling fixed by this patch.

Without this patch, FileCheck behaves as follows:

```
$ cat check
CHECK-NOT: [[#0x8000000000000000+0x8000000000000000]]

$ FileCheck -vv -dump-input=never check < input
check:1:54: remark: implicit EOF: expected string found in input
CHECK-NOT: [[#0x8000000000000000+0x8000000000000000]]
                                                     ^
<stdin>:2:1: note: found here

^
check:1:15: error: unable to substitute variable or numeric expression: overflow error
CHECK-NOT: [[#0x8000000000000000+0x8000000000000000]]
              ^
$ echo $?
0
```

Notice that the exit status is 0 even though there's an error.
Moreover, FileCheck doesn't print the error diagnostic unless both
`-dump-input=never` and `-vv` are specified.

The same problem occurs when `CHECK-NOT` does have a match but a
capture fails ...