Put std lib analysis data in per-target directories

[nrc]

This is probably a Rustbuild/Rustup/rls-analysis issue, but tracking here because I don't know exactly where to fix it.

If a user has multiple toolchains installed with rustup, then they will get multiple sets of analysis data (the JSON files) for the std libs, one per target (I think). These get installed into the same directory (e.g., /Users/nick/.rustup/toolchains/nightly-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/analysis). That means that indexing takes twice as long as it should and there is duplicate data (though I think rls-analysis will not duplicate, it might do some kind of bad merge).

[nrc]

This is not a rustup issue like I thought, but that Rustbuild is making two copies of the analysis data for at least some toolchains.