c++: print source code in print_instantiation_partial_context_line

As mentioned in my Cauldron talk, this patch adds a call to
diagnostic_show_locus to the "required from here" messages
in print_instantiation_partial_context_line, so that e.g., rather
than the rather mystifying:

In file included from ../x86_64-pc-linux-gnu/libstdc++-v3/include/memory:78,
                 from ../../src/demo-1.C:1:
../x86_64-pc-linux-gnu/libstdc++-v3/include/bits/unique_ptr.h: In instantiation of ‘std::__detail::__unique_ptr_t<_Tp> std::make_unique(_Args&& ...) [with _Tp = bar; _Args = {}; __detail::__unique_ptr_t<_Tp> = __detail::__unique_ptr_t<bar>]’:
../../src/demo-1.C:15:32:   required from here
../x86_64-pc-linux-gnu/libstdc++-v3/include/bits/unique_ptr.h:1066:30: error: no matching function for call to ‘bar::bar()’
 1066 |     { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src/demo-1.C:10:3: note: candidate: ‘bar::bar(int)’
   10 |   bar (int);
      |   ^~~
../../src/demo-1.C:10:3: note:   candidate expects 1 argument, 0 provided
../../src/demo-1.C:7:7: note: candidate: ‘constexpr bar::bar(const bar&)’
    7 | class bar : public foo
      |       ^~~
../../src/demo-1.C:7:7: note:   candidate expects 1 argument, 0 provided
../../src/demo-1.C:7:7: note: candidate: ‘constexpr bar::bar(bar&&)’
../../src/demo-1.C:7:7: note:   candidate expects 1 argument, 0 provided

we emit:

In file included from ../x86_64-pc-linux-gnu/libstdc++-v3/include/memory:78,
                 from ../../src/demo-1.C:1:
../x86_64-pc-linux-gnu/libstdc++-v3/include/bits/unique_ptr.h: In instantiation of ‘std::__detail::__unique_ptr_t<_Tp> std::make_unique(_Args&& ...) [with _Tp = bar; _Args = {}; __detail::__unique_ptr_t<_Tp> = __detail::__unique_ptr_t<bar>]’:
../../src/demo-1.C:15:32:   required from here
   15 |   return std::make_unique<bar> ();
      |          ~~~~~~~~~~~~~~~~~~~~~~^~
../x86_64-pc-linux-gnu/libstdc++-v3/include/bits/unique_ptr.h:1066:30: error: no matching function for call to ‘bar::bar()’
 1066 |     { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
      |                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src/demo-1.C:10:3: note: candidate: ‘bar::bar(int)’
   10 |   bar (int);
      |   ^~~
../../src/demo-1.C:10:3: note:   candidate expects 1 argument, 0 provided
../../src/demo-1.C:7:7: note: candidate: ‘constexpr bar::bar(const bar&)’
    7 | class bar : public foo
      |       ^~~
../../src/demo-1.C:7:7: note:   candidate expects 1 argument, 0 provided
../../src/demo-1.C:7:7: note: candidate: ‘constexpr bar::bar(bar&&)’
../../src/demo-1.C:7:7: note:   candidate expects 1 argument, 0 provided

which shows the code that's leading to the error (the bad call to
std::make_unique).

gcc/cp/ChangeLog:
	* error.cc (print_instantiation_partial_context_line): Call
	diagnostic_show_locus.

gcc/testsuite/ChangeLog:
	* g++.dg/diagnostic/static_assert3.C: Add directives for
	additional source printing.
	* g++.dg/template/error60.C: New test.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>