[clang] Tweaked fixit for static assert with no message

If a static assert has a message as the right side of an and condition, suggest a fix it of replacing the '&&' to ','.

`static_assert(cond && "Failed Cond")` -> `static_assert(cond, "Failed cond")`

This use case comes up when lazily replacing asserts with static asserts.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D89065