[clang-tools-extra] [clangd] Propagate context into stdlib indexing thread (PR #87611)

[Sam McCall via cfe-commits]

https://github.com/sam-mccall commented:

I'm iffy on whether this is semantically right or not, but we need to solve this problem somehow, so let's try the most obvious way.

https://github.com/llvm/llvm-project/pull/87611