[clang] Add `ObjCProtocolLoc` to represent protocol references

Add `ObjCProtocolLoc` which behaves like `TypeLoc` but for
`ObjCProtocolDecl` references.

RecursiveASTVisitor now synthesizes `ObjCProtocolLoc` during traversal
and the `ObjCProtocolLoc` can be stored in a `DynTypedNode`.

In a follow up patch, I'll update clangd to make use of this
to properly support protocol references for hover + goto definition.

Differential Revision: https://reviews.llvm.org/D119363