[clangd] Value initialize SymbolIDs

[clangd] Value initialize SymbolIDs

We were default initializing SymbolIDs before, which would leave
indeterminate values in underlying std::array.

This patch updates the underlying data initalization to be value-init and adds a
way to check for validness of a SymbolID.

Differential Revision: https://reviews.llvm.org/D90397