Make RE2::Set and FilteredRE2 movable.

Change-Id: I2f5d431f59fc4ba015ada3a19ba40e7e4f4df2d3
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57410
Reviewed-by: Paul Wankadia <junyer@google.com>
4 files changed