[clangd] Introduce a CommandLineConfigProvider

This enables unifying command line flags with config options in clangd
internals. This patch changes behaviour in 2 places:
- BackgroundIndex was previously disabled when -remote-index was
provided. After this patch, it will be enabled but all files will have
bkgindex policy set to Skip.
- -index-file was loaded at startup (at least load was initiated), now
the load will happen through ProjectAwareIndex with first index query.

Unfortunately this doesn't simplify any options initially, as
- CompileCommandsDir is also used by clangd --check workflow, which
doesn't use configs.
- EnableBackgroundIndex option controls whether the component will be
created at all, which implies creation of extra threads registering a
listener for compilation database discoveries.

Differential Revision: https://reviews.llvm.org/D98029