Clang optimizes away CheckOverflowForIntSub at any opt
level (includes -O1, -O2, -O3) into a false statement,
resulting in incorrect values being returned. As the C++
standard considers overflows to be undefined behaviour,
this is technically correct as compilers can assume that
overflows never occur, but problematic in our case (where
overflows do occur, and a specific result is expected).

This change replaces the original check with a call to a
function that is optimized in a manner that returns correct output.

R=michael_dawson@ca.ibm.com,jyan@ca.ibm.com,mtbrandyberry@ca.ibm.com,joransiu@ca.ibm.com,danno@chromium.org,jkummerow@chromium.org,jochen@chromium.org

BUG=

Review URL: https://codereview.chromium.org/1826043002

Cr-Commit-Position: refs/heads/master@{#35082}