Tracking issue for llvm-tools-preview becoming not preview or better documented

[carols10cents]

Tracking issue: I don't see any tracking issue for what's keeping llvm-tools in preview status. This is where it was added: rust-lang/rust#49584

Place that will need to be updated: https://github.com/rust-lang-nursery/edition-guide/blame/68c458b9b68edced39aebea60e672a22d4d50edf/src/rust-2018/rustup-for-managing-rust-versions.md#L207-L217

llvm-tools-preview for using extra LLVM tools

If you'd like to use the lld linker, or other tools like llvm-objdump or
llvm-objcopy, you can install this component:

$ rustup component add llvm-tools-preview

This is the newest component, and so doesn't have good documentation at the
moment.

[ehuss]

Closing per #232, this section has been removed. I have opened rust-lang/rust#85658 to further track the -preview nature of the component, and to create some documentation for it.