c++: print conversion error at candidate location

c++: print conversion error at candidate location

In testcases like this one, the printing of candidates in a diagnostic has
been longer than necessary because it jumps back and forth between the call
site and the candidate site.  So here, we first say at the call site that no
match was found; then we note the candidate site, and then explain why it's
not suitable back at the call site, which means printing the call site line
with caret again.  With this patch, the conversion diagnostic is at the same
location as the candidate, so we don't need to print any input line.

gcc/cp/ChangeLog:

	* call.cc (print_conversion_rejection): Use iloc_sentinel.

gcc/testsuite/ChangeLog:

	* g++.dg/template/copy1.C: Adjust error lines.