[clangd] Improve const-correctness of Symbol->Detail. NFC

Summary:
This would have caught a bug I wrote in an early version of D42049, where
an index user could overwrite data internal to the index because the Symbol is
not deep-const.

The YAML traits are now a bit more verbose, but separate concerns a bit more
nicely: ArenaPtr can be reused for other similarly-allocated objects, including
scalars etc.

Reviewers: hokein

Subscribers: klimek, ilya-biryukov, cfe-commits, ioeric

Differential Revision: https://reviews.llvm.org/D42059

llvm-svn: 322509