[clang-tools-extra] [clangd] Store documentation when indexing standard library (PR #133681)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

See diagnosis [here](https://github.com/clangd/clangd/issues/2344#issuecomment-2765249487):

> The **actual** reason that `std::vector::size` is missing documentation when the standard library AST is indexed is that, unlike during preamble indexing, `Opts.StoreAllDocumentation` is false [here](https://searchfox.org/llvm/rev/10dd404d9fbe505fb189972565c577318a6b577b/clang-tools-extra/clangd/index/SymbolCollector.cpp#1098).

(and additional details earlier in the thread).

https://github.com/llvm/llvm-project/pull/133681