[clangd] UI for completion items that would trigger include insertion.

Summary:
For completion items that would trigger include insertions (i.e. index symbols
that are not #included yet), add a visual indicator "+" before the completion
label. The inserted headers will appear in the completion detail.

Open to suggestions for better visual indicators; "+" was picked because it
seems cleaner than a few other candidates I've tried (*, #, @ ...).

The displayed header would be like a/b/c.h (without quote) or <vector> for system
headers. I didn't add quotation or "#include" because they can take up limited
space and do not provide additional information after users know what the
headers are. I think a header alone should be obvious for users to infer that
this is an include header..

To align indentation, also prepend ' ' to labels of candidates that would not
trigger include insertions (only for completions where index results are
possible).

Vim:
{F6357587}

vscode:
{F6357589}
{F6357591}

Reviewers: sammccall, ilya-biryukov, hokein

Reviewed By: sammccall

Subscribers: MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D48163

llvm-svn: 334828