[libclang] Add `CXSymbolRole role` to CXIdxEntityRefInfo

Summary:
CXIdxEntityRefInfo contains the member `CXIdxEntityRefKind kind;` to
differentiate implicit and direct calls. However, there are more roles
defined in SymbolRole. Among them, `Read/Write` are probably the most
useful ones as they can be used to differentiate Read/Write occurrences
of a symbol for document highlight in a text document.

See `export namespace DocumentHighlightKind`
on https://microsoft.github.io/language-server-protocol/specification

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42895

llvm-svn: 324914