ada-lex.l: Ignore register diagnostic also for g++ defaulting to ISO C++17

Building with a really old flex and a really new g++ is probably not
recommended, but it should not cause compile errors.

gdb/ChangeLog:

	* ada-lex.l: Extend register warnings diagnostics comment for g++.

include/ChangeLog:

	* diagnostics.h (DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER): Also define
	for GCC version 7.0 or higher.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2020-08-21  Mark Wielaard  <mark@klomp.org>
+
+	* ada-lex.l: Extend register warnings diagnostics comment for g++.
+
 2020-08-22  Simon Marchi  <simon.marchi@efficios.com>
 
 	* frame.c (enum class frame_id_status): New.

gdb/ada-lex.l

@@ -45,7 +45,9 @@ POSEXP  (e"+"?{NUM10})
 
 /* Some old versions of flex generate code that uses the "register" keyword,
    which clang warns about.  This was observed for example with flex 2.5.35,
-   as shipped with macOS 10.12.  */
+   as shipped with macOS 10.12.  The same happens with flex 2.5.37 and g++ 11
+   which defaults to ISO C++17, that does not allow register storage class
+   specifiers.  */
 DIAGNOSTIC_PUSH
 DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER
 

include/ChangeLog

@@ -1,3 +1,8 @@
+2020-08-21  Mark Wielaard  <mark@klomp.org>
+
+	* diagnostics.h (DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER): Also define
+	for GCC 7.0 or higher.
+
 2020-08-10  Alex Coplan  <alex.coplan@arm.com>
 
 	* opcode/aarch64.h (AARCH64_MAX_SYSREG_NAME_LEN): New.

include/diagnostics.h

@@ -63,6 +63,11 @@
 
 #elif defined (__GNUC__) /* GCC */
 
+# if __GNUC__ >= 7
+#  define DIAGNOSTIC_IGNORE_DEPRECATED_REGISTER \
+   DIAGNOSTIC_IGNORE ("-Wregister")
+# endif
+
 # define DIAGNOSTIC_IGNORE_STRINGOP_TRUNCATION \
   DIAGNOSTIC_IGNORE ("-Wstringop-truncation")