[clangd] Collect symbol occurrences in SymbolCollector.

SymbolCollector will be used for two cases:
 - collect Symbol type only, used for indexing preamble AST.
 - collect Symbol and SymbolOccurrences, used for indexing main AST.

For finding local references from the AST, we will implement it in other ways.

llvm-svn: 341208