DMD front end should define a version containing the front end version

[Nick Sabalausky]

http://dlang.org/phobos/std_compiler.html#Vendor
http://dlang.org/phobos/std_compiler.html#version_major

Although it might be nice if GDC/LDC carried the version number of the
DMDFE it was based on (If that would be applicable. I don't know if
they're always based on an exact release version of DMDFE.)