Addresses failures with the following, in some files:
-Wshorten-64-to-32
-Wimplicit-int-conversion
-Wsign-compare
-Wsign-conversion
-Wtautological-unsigned-zero-compare

(This specific CL focuses on .cc files in strings/internal/.)

Bug: chromium:1292951
PiperOrigin-RevId: 470810568
Change-Id: Ibd316a7e62cc43cb198ba22daed565c9573ce235