[clang] Add size filter for stack auto init (#74777)

Add a clang flag, "-ftrivial-auto-var-init-max-size=" so that clang
skips auto-init a variable if the auto-init memset size exceeds the flag
setting (in bytes). Note that this skipping doesn't apply to
runtime-sized variables like VLA.

Considerations: "__attribute__((uninitialized))" can be used to manually
opt variables out. However, there are thousands of large variables
(e.g., >=1KB, most of them are arrays and used as buffers) in big
codebase. Manually opting them out one by one is not efficient.