[Tracking issue] rust-analyzer in-tree build prerequisites

[fasterthanlime]

The rationale for "why move from submodule to subtree" is in:

• Move rust-analyzer from a git submodule to a git subtree #12815

The proof of concept PR is here:

• Re-add rust-analyzer as a subtree rust#99465

This is a list of what needs to happen before rust-analyzer the real rust-lang/rust PR to re-add it as a subtree can happen:

• Enable (and fix) extra lint groups required for in-tree build #12826
• Rename proc macro server from 'Rustc' to 'RustAnalyzer' #12828
• Remove check_merge_commits test #12829
• Build proc-macro-test-impl out-of-tree #12831
• Add proc-macro-srv integration test that clones literals #12833
• Add PROC_MACRO_TEST_TOOLCHAIN environment variable #12834
• Introduce proc-macro-srv/sysroot-abi #12835

I'll expand the list until the research PR passes in CI, and try to land all these changes as separate rust-analyzer PRs.