Silence -Wswitch-enum from bison generated file (#4886)

p4parsers.ypp generates a .hpp file that contains an empty switch
statement with just a default case when generating the user destructor.
This switch prevents builds with -Werror=switch-enum. Silence this
warning if the flag is present.

Signed-off-by: Chris Dodd <cdodd@nvidia.com>
Co-authored-by: Emre Orbay <eorbay@nvidia.com>

frontends/parsers/p4/p4parser.ypp

@@ -48,12 +48,17 @@ limitations under the License.
 
 // Ignore a warning caused by autogenerated code.
 // The #if pragma is a little awkward because some preprocessors do not like ||
+#pragma GCC diagnostic push
 #if defined(__has_warning)
 #if __has_warning("-Wunused-but-set-variable")
 #pragma GCC diagnostic ignored "-Wunused-but-set-variable"
 #endif
+#if __has_warning("-Wswitch-enum")
+#pragma GCC diagnostic ignored "-Wswitch-enum"
+#endif
 #else
 #pragma GCC diagnostic ignored "-Wunused-but-set-variable"
+#pragma GCC diagnostic ignored "-Wswitch-enum"
 #endif
 
 namespace P4 {
@@ -1713,3 +1718,5 @@ void P4Parser::error(const Util::SourceInfo& location,
 }
 
 }  // namespace P4
+
+#pragma GCC diagnostic pop