Add DECL_SOURCE_COLUMN.

I'm currently writing a plugin for GCC and I need to extract the
column of a declaration. There exists DECL_SOURCE_FILE/LINE already
and for consistency reasons and ease of use I've added a
DECL_SOURCE_COLUMN macro.

2012-06-20  Rüdiger Sonderfeld  <ruediger@c-plusplus.de>

	* tree.h (DECL_SOURCE_COLUMN): New accessor

From-SVN: r188841