[clangd] Add $/memoryUsage LSP extension

Performs a detailed profiling of clangd lsp server and conveys the
result to the client as a json object. It is of the form:
   {
     "_self": 0,
     "_total": 8,
     "child1": {
       "_self": 4,
       "_total": 4,
     }
     "child2": {
       "_self": 2,
       "_total": 4,
       "child_deep": {
         "_self": 2,
         "_total": 2,
       }
     }
   }

Differential Revision: https://reviews.llvm.org/D89277