Remove redundant passing around of a "ContainsAutoType" flag.

This flag serves no purpose other than to prevent us walking through a type to
check whether it contains an 'auto' specifier; this duplication of information
is error-prone, does not appear to provide any performance benefit, and will
become less practical once we support C++1z deduced class template types and
eventually constrained types from the Concepts TS.

No functionality change intended.

llvm-svn: 291737