[clang] [clang][CodeGen] Zero init unspecified fields in initializers in C (PR #97121)

[via cfe-commits]

yabinc wrote:

According to the C23 standard, initializing a union variable with empty braces {} only guarantees the initialization of the first member and any padding after all members. This behavior deviates from common intuition and caused a bug when compiling the Linux kernel.
Based on the discussion in https://github.com/llvm/llvm-project/issues/78034, this patch aims to zero-initialize all bytes after the first member.
If this change introduces significant regressions in edge cases, a fallback strategy is to replace brace initialization with explicit assignment statements for the specific fields requiring initialization.

https://github.com/llvm/llvm-project/pull/97121