Improved (line number, file name) arguments inside chpl_comm_wide_get_string.

[pre-reviewed by Greg]

chpl_comm_wide_get_string used to have:

      chpl_mem_calloc(x->size, CHPL_RT_MD_GET_WIDE_STRING, -1, "<internal>");

But (line number, file name) are available as chpl_comm_wide_get_string's
formals. So use them instead.


git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21884 3a8e244f-b0f2-452b-bcba-4c88e055c3ca