[clangd] Add decl/def support to SymbolDetails

[clangd] Add decl/def support to SymbolDetails

Add an optional declarationRange and definitionRange to SymbolDetails.

This will allow SourceKit-LSP to implement toggling between goto
definition/declaration based on whether the symbol at the cursor
is a definition or declaration.

Differential Revision: https://reviews.llvm.org/D130041