[clang][extract-api] Add support for typedefs

Typedef records consist of the symbol associated with the underlying
TypedefDecl and a SymbolReference to the underlying type. Additionally
typedefs for anonymous TagTypes use the typedef'd name as the symbol
name in their respective records and USRs. As a result the declaration
fragments for the anonymous TagType are those for the associated
typedef. This means that when the user is defining a typedef to a
typedef to a anonymous type, we use a reference the anonymous TagType
itself and do not emit the typedef to the anonymous type in the
generated symbol graph, including in the type destination of further
typedef symbol records.

Differential Revision: https://reviews.llvm.org/D123019