[clangd] Implement textDocument/hover

Summary: Implemention of textDocument/hover as described in LSP definition.

This patch adds a basic Hover implementation.  When hovering a variable,
function, method or namespace, clangd will return a text containing the
declaration's scope, as well as the declaration of the hovered entity.
For example, for a variable:

  Declared in class Foo::Bar

  int hello = 2

For macros, the macro definition is returned.

This patch doesn't include:

- markdown support (the client I use doesn't support it yet)
- range support (optional in the Hover response)
- comments associated to variables/functions/classes

They are kept as future work to keep this patch simpler.

I added tests in XRefsTests.cpp.  hover.test contains one simple
smoketest to make sure the feature works from a black box perspective.

Reviewers: malaperle, krasimir, bkramer, ilya-biryukov

Subscribers: sammccall, mgrang, klimek, rwols, ilya-biryukov, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D35894



Signed-off-by: Simon Marchi <simon.marchi@ericsson.com>
Signed-off-by: William Enright <william.enright@polymtl.ca>
llvm-svn: 325395