[clangd] Avoid inserting new #include when declaration is present in the main file.

Summary:
Also fix USR generation for classes in unit tests. The previous USR
only works for class members, which happens to work when completing class name
inside the class, where constructors are suggested by sema.

Reviewers: sammccall, ilya-biryukov

Subscribers: klimek, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47466

llvm-svn: 333519