Add LSP extension to show Macro Expansions (or any document) in a "peeked" editor (and some minor quality improvements)#1479

ahoppen merged 2 commits intoswiftlang:mainswiftlang/sourcekit-lsp:mainfrom

lokesh-tr:show-attached-macro-expansionslokesh-tr/sourcekit-lsp:show-attached-macro-expansionsCopy head branch name to clipboard