[AST] Use written names instead of `type-param-0-0` in more cases when printing types and decls.

Summary:
This doesn't cover decls in diagnostics, which use NamedDecl::getNameForDiagnostic().
(That should also be fixed later I think).

This covers some cases of https://github.com/clangd/clangd/issues/76
(hover, but not outline or sighelp)

Reviewers: hokein

Subscribers: ilya-biryukov, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70236