c++: Fix poor diagnostic for array initializer [PR93710]

c++: Fix poor diagnostic for array initializer [PR93710]

A small improvement for an error in build_user_type_conversion_1:
instead of

array-init1.C:11:1: error: conversion from ‘long int’ to ‘A’ is ambiguous
   11 | };
      | ^

we will print

array-init1.C:8:3: error: conversion from ‘long int’ to ‘A’ is ambiguous
    8 |   0L,
      |   ^~

2020-02-12  Marek Polacek  <polacek@redhat.com>

	PR c++/93710 - poor diagnostic for array initializer.
	* call.c (build_user_type_conversion_1): Use cp_expr_loc_or_input_loc
	for an error call.

	* g++.dg/diagnostic/array-init1.C: New test.