[clangd] Switch xrefs and documenthighlight to annotated-code unit tests. NFC

Summary:
The goal here is again to make it easier to read and write the tests.

I've extracted `parseTextMarker` from CodeCompleteTests into an `Annotations`
class, adding features to it:
  - as well as points `^s` it allows ranges `[[...]]`
  - multiple points and ranges are supported
  - points and ranges may be named: `$name^` and `$name[[...]]`

These features are used for the xrefs tests. This also paves the way for
replacing the lit diagnostics.test with more readable unit tests, using named
ranges.

Alternative considered: `TestSelectionRange` in clang-refactor/TestSupport
Main problems were:
 - delimiting the end of ranges is awkward, requiring counting
 - comment syntax is long and at least as cryptic for most cases
 - no separate syntax for point vs range, which keeps xrefs tests concise
 - Still need to convert to Position everywhere
 - Still need helpers for common case of expecting exactly one point/range

(I'll probably promote the extra `PrintTo`s from some of the core Protocol types
into `operator<<` in `Protocol.h` itself in a separate, prior patch...)

Reviewers: ioeric

Subscribers: klimek, mgorny, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D41432

llvm-svn: 321184