Fix a few auto-related issues:

 * 'auto' was being rejected on abstract-declarators with trailing return
types and on typedefs with trailing return types. 'auto' is always
allowed in these cases. This was found while testing the fix for PR 9278.

 * A very poor diagnostic was being issued for auto (f() -> int): "return
type must be 'auto', not 'auto'". This is closely related to PR 9060.

 * Trailing return type handling was happening slightly too late,
resulting in the checks for functions returning arrays and functions
returning functions being missed.

llvm-svn: 126166