gdb, include: replace pragmas with DIAGNOSTIC macros, fix build with g++ 4.8

When introducing this code, I forgot that we had some macros for this.
Replace some "manual" pragma diagnostic with some DIAGNOSTIC_* macros,
provided by include/diagnostics.h.

In diagnostics.h:

 - Add DIAGNOSTIC_ERROR, to enable a diagnostic at error level.
 - Add DIAGNOSTIC_ERROR_SWITCH, to enable -Wswitch at error level, for
   both gcc and clang.

Additionally, using DIAGNOSTIC_PUSH, DIAGNOSTIC_ERROR_SWITCH and
DIAGNOSTIC_POP seems to misbehave with g++ 4.8, where we see these
errors:

      CXX    ada-tasks.o
    /home/smarchi/src/binutils-gdb/gdb/ada-tasks.c: In function void read_known_tasks():
    /home/smarchi/src/binutils-gdb/gdb/ada-tasks.c:998:10: error: enumeration value ADA_TASKS_UNKNOWN not handled in switch [-Werror=switch]
       switch (data->known_tasks_kind)
              ^

Because of the POP, the diagnostic should go back to being disabled,
since it was disabled in the beginning, but that's not what we see
here.  Versions of GCC >= 5 compile correctly.

Work around this by making DIAGNOSTIC_ERROR_SWITCH a no-op for GCC < 5.

Note that this code (already as it exists in master today) enables
-Wswitch at the error level even if --disable-werror is passed.  It
shouldn't be a problem, as it's not like a new enumerator will appear
out of nowhere and cause a build error if building with future
compilers.  Still, for correctness, we would ideally want to ask the
compiler to enable -Wswitch at its default level (as if the user had
passed -Wswitch on the command-line).  There doesn't seem to be a way to
do this.

Change-Id: Id33ebec3de39bd449409ea0bab59831289ffe82d

include/diagnostics.h

@@ -40,6 +40,8 @@
 
 # define DIAGNOSTIC_IGNORE(option) \
   _Pragma (DIAGNOSTIC_STRINGIFY (GCC diagnostic ignored option))
+# define DIAGNOSTIC_ERROR(option) \
+  _Pragma (DIAGNOSTIC_STRINGIFY (GCC diagnostic error option))
 #else
 # define DIAGNOSTIC_PUSH
 # define DIAGNOSTIC_POP
@@ -61,6 +63,9 @@
 # define DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL \
   DIAGNOSTIC_IGNORE ("-Wformat-nonliteral")
 
+# define DIAGNOSTIC_ERROR_SWITCH \
+  DIAGNOSTIC_ERROR ("-Wswitch")
+
 #elif defined (__GNUC__) /* GCC */
 
 # if __GNUC__ >= 7
@@ -74,6 +79,13 @@
 # define DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL \
   DIAGNOSTIC_IGNORE ("-Wformat-nonliteral")
 
+/* GCC 4.8's "diagnostic push/pop" seems broken when using this, -Wswitch
+   remains enabled at the error level even after a pop.  Therefore, don't
+   use it for GCC < 5.  */
+# if __GNUC__ >= 5
+#  define DIAGNOSTIC_ERROR_SWITCH DIAGNOSTIC_ERROR ("-Wswitch")
+# endif
+
 #endif
 
 #ifndef DIAGNOSTIC_IGNORE_SELF_MOVE
@@ -100,4 +112,8 @@
 # define DIAGNOSTIC_IGNORE_FORMAT_NONLITERAL
 #endif
 
+#ifndef DIAGNOSTIC_ERROR_SWITCH
+# define DIAGNOSTIC_ERROR_SWITCH
+#endif
+
 #endif /* DIAGNOSTICS_H */