Adds a warning when an inline Doxygen comment has no argument

Summary:
It warns for for comments like
/** \pre \em */

where \em has no argument

This warning is enabled with the -Wdocumentation option.

Reviewers: gribozavr, rsmith

Reviewed By: gribozavr

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64696

Patch by Mark de Wever.

llvm-svn: 367809