Configure a build matrix for Clang using containers.
Configure a build matrix for Clang using containers.

Change-Id: I8dbc53f3a45f1f1f215e8c949275b85fdef2c8e3
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57800
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2