[clangd] NFC: Cleanup Dex Iterator comments and simplify tests

Proposed changes:

* Cleanup comments in `clangd/index/dex/Iterator.h`: Vim's `gq`
  formatting added redundant spaces instead of newlines in few
  places
* Few comments in `OrIterator` are wrong
* Use `EXPECT_TRUE(Condition)` instead of
  `EXPECT_THAT(Condition, true)` (same with `EXPECT_FALSE`)
* Don't expose `dump()` method to the public by misplacing
  `private:`

This patch does not affect functionality.

Reviewed by: ioeric

Differential Revision: https://reviews.llvm.org/D50956

llvm-svn: 340157