[clang] Refactor doc comments to Decls attribution

- Create ASTContext::attachCommentsToJustParsedDecls so we don't have to load external comments in Sema when trying to attach existing comments to just parsed Decls.
- Keep comments ordered and cache their decomposed location - faster SourceLoc-based searching.
- Optimize work with redeclarations.
- Keep one comment per redeclaration chain (represented by canonical Decl) instead of comment per redeclaration.
- For redeclaration chains with no comment attached keep just the last declaration in chain that had no comment instead of every comment-less redeclaration.

Differential Revision: https://reviews.llvm.org/D65301

llvm-svn: 368732