No implementation uses the LocCookie parameter at all. Errors are
reported from inside that function by llvm::SourceMgr, and the
instance of that at the clang call site arranges to pass the error
messages back to a ClangAsmParserCallback, which is where the clang
SourceLocation for the error is computed.
(This is part of a patch series working towards the ability to make
SourceLocation into a 64-bit type to handle larger translation units.
But this particular change seems beneficial in its own right.)