c++: locations: provide convenience constructors.

c++: locations: provide convenience constructors.

* data/location.cc (position::position): Accept file, line and
column as arguments with default values.
Always qualify initial line and column literals as unsigned.
(location::location): Provide convenience constructors.
(cherry picked from commit 0634493cdde92cbdb8d9fb95a5bebe596bb53798)

Conflicts:

	ChangeLog-2012
	data/location.cc