[clangd] Add "str()" method to SymbolID.

Summary:
This is a convenient function when we try to get std::string of
SymbolID.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D46065

llvm-svn: 330835