[clang] [RFC][clang][BPF] Make trivial uninit var value to be 0 (PR #125601)

[via cfe-commits]

4ast wrote:

what veristat says before/after?

re: selftests

there should be a flag to disable this, so the tests can remain as-is (with only Makefile change).

There is also -ftrivial-auto-var-init-max-size= flag. What is the default?
Looks like: char buf[64]; will be zero inited as well?
That will probably hurt performance/verification for cases like:
char comm[16];
bpf_get_current_comm(comm, ...)
?

https://github.com/llvm/llvm-project/pull/125601