Try once again to fix the MSVC build of AlignedCharArrayUnion

It was complaining about ambiguity between llvm::detail and
llvm::support::detail:
  error C2872: 'detail': ambiguous symbol
  note: could be 'llvm::detail'
  note: or       'llvm::support::detail'

Standardize on llvm::support::detail to hide these symbols further.

llvm-svn: 291519