C++: fix portability issue with MSVC 2017

Visual Studio issues a C4146 warning on '-static_cast<unsigned>(rhs)'.
The code is weird, probably to cope with INT_MIN.  Let's go back to
using std::max (whose header is still included in position.hh...) like
originally, but with the needed casts.

Reported by 長田偉伸, and with help from Rici Lake.

See also
http://lists.gnu.org/archive/html/bug-bison/2013-02/msg00000.html
and commit 75ae829984.

* data/location.cc (position::add_): Take min as an int.
Use std::max.
While here, get rid of a couple of useless inlines.

THANKS

@@ -172,6 +172,7 @@ Wolfram Wagner            ww@mpi-sb.mpg.de
 Wwp                       subscript@free.fr
 xolodho                   xolodho@gmail.com
 Zack Weinberg             zack@codesourcery.com
+長田偉伸                   cbh34680@iret.co.jp
 
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