[clang] Add '-ast-dump-filter=' support

Before this patch, we only support syntax like
`clang -cc1 -ast-dump -ast-dump-filter main a.c`
or
`clang -Xclang -ast-dump -Xclang -ast-dump-filter -Xclang main a.c`
when using ast-dump-filter.

It is helpful to also support `-ast-dump-filter=` syntax, so we can do
something like
`clang -cc1 -ast-dump -ast-dump-filter=main a.c`
or
`clang -Xclang -ast-dump -Xclang -ast-dump-filter=main a.c`

It is more cleaner when passing arguments through `-Xclang` in this case.

Also, **clang-check** do support this syntax, and I think people might
be confiused when they found they can't use `ast-dump-filter` with
clang.