c++: fix source printing for "required from here" message

c++: fix source printing for "required from here" message

It seems the diagnostic machinery's source line printing respects
the pretty printer prefix, but this is undesirable for the call to
diagnostic_show_locus in print_instantiation_partial_context_line
(added in r14-4388-g1c45319b66edc9) since the prefix may have been
set when issuing an earlier, unrelated diagnostic and we just want
to print an unprefixed source line.

This patch naively fixes this by clearing the prefix before calling
diagnostic_show_locus.

Before this patch, for error60a.C below we'd print

gcc/testsuite/g++.dg/template/error60a.C: In function ‘void usage()’:
gcc/testsuite/g++.dg/template/error60a.C:24:3: error: ‘unrelated_error’ was not declared in this scope
   24 |   unrelated_error; // { dg-error "not declared" }
      |   ^~~~~~~~~~~~~~~
gcc/testsuite/g++.dg/template/error60a.C: In instantiation of ‘void test(Foo) [with Foo = int]’:
gcc/testsuite/g++.dg/template/error60a.C:25:13:   required from here
gcc/testsuite/g++.dg/template/error60a.C:24:3: error:    25 |   test<int> (42); // { dg-message " required from here" }
gcc/testsuite/g++.dg/template/error60a.C:24:3: error:       |   ~~~~~~~~~~^~~~
gcc/testsuite/g++.dg/template/error60a.C:19:24: error: invalid conversion from ‘int’ to ‘int*’ [-fpermissive]
   19 |   my_pointer<Foo> ptr (val); // { dg-error "invalid conversion from 'int' to 'int\\*'" }
      |                        ^~~
      |                        |
      |                        int
gcc/testsuite/g++.dg/template/error60a.C:9:20: note:   initializing argument 1 of ‘my_pointer<Foo>::my_pointer(Foo*) [with Foo = int]’
    9 |   my_pointer (Foo *ptr) // { dg-message " initializing argument 1" }
      |               ~~~~~^~~

and afterward we print

gcc/testsuite/g++.dg/template/error60a.C: In function ‘void usage()’:
gcc/testsuite/g++.dg/template/error60a.C:24:3: error: ‘unrelated_error’ was not declared in this scope
   24 |   unrelated_error; // { dg-error "not declared" }
      |   ^~~~~~~~~~~~~~~
gcc/testsuite/g++.dg/template/error60a.C: In instantiation of ‘void test(Foo) [with Foo = int]’:
gcc/testsuite/g++.dg/template/error60a.C:25:13:   required from here
   25 |   test<int> (42); // { dg-message " required from here" }
      |   ~~~~~~~~~~^~~~
gcc/testsuite/g++.dg/template/error60a.C:19:24: error: invalid conversion from ‘int’ to ‘int*’ [-fpermissive]
   19 |   my_pointer<Foo> ptr (val); // { dg-error "invalid conversion from 'int' to 'int\\*'" }
      |                        ^~~
      |                        |
      |                        int
gcc/testsuite/g++.dg/template/error60a.C:9:20: note:   initializing argument 1 of ‘my_pointer<Foo>::my_pointer(Foo*) [with Foo = int]’
    9 |   my_pointer (Foo *ptr) // { dg-message " initializing argument 1" }
      |               ~~~~~^~~

gcc/cp/ChangeLog:

	* error.cc (print_instantiation_partial_context_line): Clear the
	pretty printer prefix around the call to diagnostic_show_locus.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic2.C: Expect source line printed
	for the "required from here" message.
	* g++.dg/template/error60a.C: New test.