[clangd] give project config higher precedence than user config

[khuldraeseth]

Changing the order of the provider stack to place project config higher lets a project config override elements of the user config rather than the other way around.

Discussed in clangd/clangd#1193. I've been using a build with this change off and on since asking about it there and haven't had any trouble from it. Any reason to resist this change - some rationale behind the current behavior that I've overlooked, or something that won't work right with just this +2/-2 diff?

created this PR as a draft, going to leave it that way until I have a PR open in llvm/clangd-www to update the docs accordingly.

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