AST: Implement proposal for dependent elaborated type specifiers

cxx-abi-dev came up with a way to disambiguate between different
keywords used in elaborated type specifiers.

This resolves certain collisions during mangling.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205943 91177308-0d34-0410-b5e6-96231b3b80d8