[FileCheck] Default --allow-unused-prefixes to false

Link: https://lists.llvm.org/pipermail/llvm-dev/2020-October/146162.html "[RFC] FileCheck: (dis)allowing unused prefixes"

If a downstream project using lit needs time for transition,
add the following to `lit.local.cfg`:

```
from lit.llvm.subst import ToolSubst

fc = ToolSubst('FileCheck', unresolved='fatal')
config.substitutions.insert(0, (fc.regex, 'FileCheck --allow-unused-prefixes'))
```

Differential Revision: https://reviews.llvm.org/D95849