Clean up a large number of C++11 attribute parse issues, including parsing

Clean up a large number of C++11 attribute parse issues, including parsing
attributes in more places where we didn't and catching a lot more issues.

This implements nearly every aspect of C++11 attribute parsing, except for:
 - Attributes are permitted on explicit instantiations inside the declarator
   (but not preceding the decl-spec)
 - Attributes are permitted on friend declarations of functions.
 - Multiple instances of the same attribute in an attribute-list (e.g.
   [[noreturn, noreturn]], not [[noreturn]] [[noreturn]] which is conforming)
   are allowed.
The first two are marked as expected-FIXME in the test file and the latter
is probably a defect and is currently untested.

Thanks to Richard Smith for providing the lion's share of the testcases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@159072 91177308-0d34-0410-b5e6-96231b3b80d8