[clang-tools-extra] [clangd] Augment code completion results with documentation from the index. (PR #120099)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> I'm not entirely sure how to add to the tests for clangd. Any advice?

We have GTest unit tests for code completion in `CodeCompleteTests.cpp`. [This test](https://searchfox.org/llvm/rev/2df48fa78b496a2d276aa848598634bb2aad6857/clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp) is a good starting point -- the test cases we want would be pretty similar, except we want to test class members (and we don't care whether it's in a system header specifically).

https://github.com/llvm/llvm-project/pull/120099