[llvm][Support] Add isZero method for TypeSize. [NFC]

Summary:
The method is used where TypeSize is implicitly cast to integer for
being checked against 0.

Reviewers: sdesmalen, efriedma

Reviewed By: sdesmalen, efriedma

Subscribers: efriedma, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76748