[FileCheck] Permit multiple -v or -vv

`FILECHECK_OPTS` was implemented so that a test runner, such as CI,
can specify FileCheck debugging options, such as `-v` and `-vv`.
However, if a test suite has a FileCheck call that already specifies
`-v` or `-vv`, then that call will fail if `FILECHECK_OPTS` also
specifies it.

For `-vv`, this problem already exists:

`clang/test/CodeGen/aarch64-v8.2a-fp16-intrinsics-constrained.c`

It's not yet clear if the `-vv` in that test was intentional, but this
usage shouldn't fail anyway.  It's already true that FileCheck permits
`-vv` and `-v` together even though `-vv` implies `-v`.

Compare D70784, which fixed the same problem for `-dump-input`.

Reviewed By: jhenderson, thopre

Differential Revision: https://reviews.llvm.org/D82601