[Symbol] Add DeclVendor::FindTypes

Summary:
Following up on the plan I outlined in D63622, we can remove the
dependence on clang in all the places where we only want to find the
types from the DeclVendor. This means that currently DeclVendor depends
on clang, but centralizing the dependency makes it easier to refactor
cleanly.

Differential Revision: https://reviews.llvm.org/D63853

llvm-svn: 364962