[clangd] Use operator<< to prevent printers issues in Gtest

Summary:
It is possible that there will be two different instantiations of
the printer template for a given type and some tests could end up calling the
wrong (default) one. For example, it was seen in CodeCompleteTests.cpp when
printing CompletionItems that it would use the wrong printer because the default
is also instantiated in ClangdTests.cpp.

With this change, objects that were previously printed with a custom Printer now
get printed through the operator<< which is declared alongside the class.
This rule of the thumb should make it less error-prone.

Reviewers: simark, ilya-biryukov, sammccall

Reviewed By: simark, ilya-biryukov, sammccall

Subscribers: bkramer, hokein, sammccall, klimek, ilya-biryukov, jkorous-apple, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D44764

llvm-svn: 329725