Enable whole-program vtable opt when CFI is enabled.

Following the roll to clang r273760, the compiler can now support
both of these features simultaneously.

Discussion thread for CFI: https://groups.google.com/a/chromium.org/d/msg/chromium-dev/pbJqt6ccMII/7iJC2oklCAAJ

BUG=464797,580389
R=thakis@chromium.org,krasin@chromium.org

Review-Url: https://codereview.chromium.org/2106313002
Cr-Commit-Position: refs/heads/master@{#403599}
2 files changed