gdb/unittests: PR28413, suppress warnings generated by Gnulib

Gnulib generates a warning if the system version of certain functions
are used (to redirect the developer to use Gnulib version).  It caused a
compiler error when...

-   Compiled with Clang
-   -Werror is specified (by default)
-   C++ standard used by Clang is before C++17 (by default as of 15.0.0)
    when this unit test is activated.

This issue is raised as PR28413.

However, previous proposal to fix this issue (a "fix" to Gnulib):
<https://lists.gnu.org/archive/html/bug-gnulib/2021-10/msg00003.html>
was rejected because it ruins the intent of Gnulib warnings.

So, we need a Binutils/GDB-side solution.

This commit tries to address this issue on the GDB side.  We have
"include/diagnostics.h" to disable certain warnings only when necessary.

This commit suppresses the Gnulib warnings by surrounding entire #include
block with DIAGNOSTIC_IGNORE_USER_DEFINED_WARNINGS to disable Gnulib-
generated warnings on all standard C++ header files.

gdb/unittests/string_view-selftests.c

@@ -23,6 +23,11 @@
 
 #define GNULIB_NAMESPACE gnulib
 
+#include "diagnostics.h"
+
+DIAGNOSTIC_PUSH
+DIAGNOSTIC_IGNORE_USER_DEFINED_WARNINGS
+
 #include "defs.h"
 #include "gdbsupport/selftest.h"
 #include "gdbsupport/gdb_string_view.h"
@@ -34,6 +39,8 @@
 #include <fstream>
 #include <iostream>
 
+DIAGNOSTIC_POP
+
 /* libstdc++'s testsuite uses VERIFY.  */
 #define VERIFY SELF_CHECK