preprocessor: Fix negative shift with bad ids.

Fix this by producing an error on undefined or negative shifts.

BUG=629518

Change-Id: Idfca5ed3fc8e557f6178408f3426a5ef2ce7cf14
Reviewed-on: https://chromium-review.googlesource.com/362020
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/compiler/preprocessor/DiagnosticsBase.h b/src/compiler/preprocessor/DiagnosticsBase.h
index d26c174..a089920 100644
--- a/src/compiler/preprocessor/DiagnosticsBase.h
+++ b/src/compiler/preprocessor/DiagnosticsBase.h
@@ -65,6 +65,7 @@
         PP_INVALID_FILE_NUMBER,
         PP_INVALID_LINE_DIRECTIVE,
         PP_NON_PP_TOKEN_BEFORE_EXTENSION_ESSL3,
+        PP_UNDEFINED_SHIFT,
         PP_ERROR_END,
 
         PP_WARNING_BEGIN,