Add support for various C++14 demanglings.

Mostly this includes <auto> and <decltype-auto> return values.
Additionally, this fixes a fairly obscure back-referencing bug
that was encountered in one of the C++14 tests, which is that
if you have something like Foo<&bar, &bar> then the `bar`
forms a backreference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340896 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed