Rvalue references for *this: add name mangling for ref-qualifiers,

Rvalue references for *this: add name mangling for ref-qualifiers,
using rules that I just made up this morning. This encoding has now
been proposed to the Itanium C++ ABI group for inclusion, but of
course it's still possible that the mangling will change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124296 91177308-0d34-0410-b5e6-96231b3b80d8