Add irony-eldoc.

It is a minor mode that enables eldoc-mode support (showing docstrings
in minibuffer) for C/C++/ObjC, based on irony-mode, which is based on
libclang. Based on irony-mode and libclang, it is quite precise (except
for ambiguous function calls), and avoids recompilation of files.

https://github.com/ikirill/irony-eldoc
Merge pull request #2322 from ikirill/add-irony-eldoc

Add irony-eldoc.