[Ada] Crash in C++ constructor without external and link name

[Ada] Crash in C++ constructor without external and link name

The compiler blows up processing the declaration of a tagged type
variable that has a C++ constructor without external or link name. After
this patch the frontend reports an error.

2019-07-22  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* freeze.adb (Freeze_Subprogram): Check that C++ constructors
	must have external or link name.

gcc/testsuite/

	* gnat.dg/cpp_constructor2.adb: New testcase.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@273670 138bc75d-0d04-0410-961f-82ee72b054a4