[MS Demangler] Create a new backref context for template instantiations.

Template manglings use a fresh back-referencing context, so we
need to do the same.  This fixes several existing tests which are
marked as FIXME, so those are now actually run.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339275 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed