Implement C++11 [expr.prim.general]p3, which permits the use of 'this'

Implement C++11 [expr.prim.general]p3, which permits the use of 'this'
in the declaration of a non-static member function after the
(optional) cv-qualifier-seq, which in practice means in the exception
specification and late-specified return type.

The new scheme here used to manage 'this' outside of a member function
scope is more general than the Scope-based mechanism previously used
for non-static data member initializers and late-parsesd attributes,
because it can also handle the cv-qualifiers on the member
function. Note, however, that a separate pass is required for static
member functions to determine whether 'this' was used, because we
might not know that we have a static function until after declaration
matching.

Finally, this introduces name mangling for 'this' and for the implicit
'this', which is intended to match GCC's mangling. Independent
verification for the new mangling test case would be appreciated.

Fixes PR10036 and PR12450.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154799 91177308-0d34-0410-b5e6-96231b3b80d8