[Tooling] Optimize memory usage in InMemoryToolResults.

Avoid storing duplicated "std::string"s.

clangd's global-symbol-builder takes 20+GB memory running across LLVM
repository. With this patch, the used memory is ~10GB (running on 48
threads, most of meory are AST-related).

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D45479

llvm-svn: 329784