[clang] [Clang][Sema]: Allow flexible arrays in unions and alone in structs (PR #84428)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

The code in question is comparing what Sema thinks the size of a variable should be, versus the size of the variable we're actually emitting into LLVM IR.  So try dumping the value of "Init".  If it looks wrong, we need to fix constant emission.  If it's right, probably something is going wrong in getFlexibleArrayInitChars().

https://github.com/llvm/llvm-project/pull/84428