Use [[fallthrough]] attribute instead of comments.

Now that we require C++17, the [[fallthrough]] attribute is available as
an alternative to load-bearing comments.

glslang/MachineIndependent/Versions.cpp

@@ -766,7 +766,7 @@ void TParseVersions::profileRequires(const TSourceLoc& loc, int profileMask, int
             switch (getExtensionBehavior(extensions[i])) {
             case EBhWarn:
                 infoSink.info.message(EPrefixWarning, ("extension " + TString(extensions[i]) + " is being used for " + featureDesc).c_str(), loc);
-                // fall through
+                [[fallthrough]];
             case EBhRequire:
             case EBhEnable:
                 okay = true;