[clangd] Indexing of standard library

This provides a nice "warm start" with all headers indexed, not just
those included so far.

The standard library is indexed after a preamble is parsed, using that
file's configuration. The result is pushed into the dynamic index.
If we later see a higher language version, we reindex it.

It's configurable as Index.StandardLibrary, off by default for now.

Based on D105177 by @kuhnel

Fixes https://github.com/clangd/clangd/issues/618

Differential Revision: https://reviews.llvm.org/D115232