[clangd] Fix bugs with incorrect memory estimate report

* With the current implementation, `sizeof(std::vector<Chunk>)` is added
twice to the `Dex` memory estimate which is incorrect
* `Dex` logs memory usage estimation before `BackingDataSize` is set and
hence the log report excludes size of the external `SymbolSlab` which is
coupled with `Dex` instance

Reviewed By: ioeric

Differential Revision: https://reviews.llvm.org/D52503

llvm-svn: 343117