Fix AST representations of alias-declarations which define tag types. Inside classes...

Fix AST representations of alias-declarations which define tag types. Inside classes, the tag types need to have an associated access specifier, and inside function definitions, they need to be included in the declarations of the DeclStmt. These issues manifested as assertions during template instantiation, and also in a WIP constexpr patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@134250 91177308-0d34-0410-b5e6-96231b3b80d8