[clangd] Fix use-after-free in SymbolYAML: strings are owned by yaml::Input!

Summary:
There are a few implementation options here - alternatives are either both
awkward and inefficient, or really inefficient.
This is at least potentially a hot path when used as a reducer for common
symbols.

(Also fix an unused-var that sneaked in)

Reviewers: ioeric

Subscribers: klimek, ilya-biryukov, jkorous-apple, cfe-commits

Differential Revision: https://reviews.llvm.org/D43381

llvm-svn: 325476