HLSL: Flesh out misc. declaration grammar: semantics/registers/annotations/precise/etc.

Details within these bear even more fleshing out, but would like to have
that driven by actual need.

hlsl/hlslScanContext.cpp

@@ -102,6 +102,7 @@ void HlslScanContext::fillInKeywordMap()
     (*KeywordMap)["extern"] =                  EHTokExtern;
     (*KeywordMap)["uniform"] =                 EHTokUniform;
     (*KeywordMap)["volatile"] =                EHTokVolatile;
+    (*KeywordMap)["precise"] =                 EHTokPrecise;
     (*KeywordMap)["shared"] =                  EHTokShared;
     (*KeywordMap)["groupshared"] =             EHTokGroupShared;
     (*KeywordMap)["linear"] =                  EHTokLinear;