Use correct enum type in case statement

Two similarly named enums led to the wrong one being used here, this
is getting caught in the Chromium and Dawn DEPS rolls.

glslang/MachineIndependent/ShaderLang.cpp

@@ -731,7 +731,7 @@ void TranslateEnvironment(const TEnvironment* environment, EShMessages& messages
                 source = EShSourceHlsl;
                 messages = static_cast<EShMessages>(messages | EShMsgReadHlsl);
                 break;
-            case EShClientCount:
+            case EShSourceCount:
                 assert(0);
                 break;
             }