[clang] [Clang][counted_by] Load the count through unions by byte offset (PR #228309)

[Michael Bommarito via cfe-commits]

mjbommar wrote:

@bwendling Confirmed on x86 7.3 rc6 that 23.1.3 is still broken without this PR, but fixed with it.  No other issues so far after a few minutes of normal use

https://github.com/llvm/llvm-project/pull/228309