Enhance readability of error messages for GLSL

Specifically, make GLSL link error messages more specific and output
only information relevant to the error.

Also change type printing to more closely reflect GLSL syntax. This
is the default for link error messages, but must me enabled with the
new option --enhanced-msgs for compilation error messages.

Also with --enhanced-msgs, only emit one error message per source
line.

StandAlone/StandAlone.cpp

@@ -178,6 +178,7 @@ const char* variableName = nullptr;
 bool HlslEnable16BitTypes = false;
 bool HlslDX9compatible = false;
 bool HlslDxPositionW = false;
+bool EnhancedMsgs = false;
 bool DumpBuiltinSymbols = false;
 std::vector<std::string> IncludeDirectoryList;
 
@@ -665,6 +666,8 @@ void ProcessArguments(std::vector<std::unique_ptr<glslang::TWorkItem>>& workItem
                         HlslDX9compatible = true;
                     } else if (lowerword == "hlsl-dx-position-w") {
                         HlslDxPositionW = true;
+                    } else if (lowerword == "enhanced-msgs") {
+                        EnhancedMsgs = true;
                     } else if (lowerword == "auto-sampled-textures") { 
                         autoSampledTextures = true;
                     } else if (lowerword == "invert-y" ||  // synonyms
@@ -1073,6 +1076,8 @@ void SetMessageOptions(EShMessages& messages)
         messages = (EShMessages)(messages | EShMsgHlslDX9Compatible);
     if (DumpBuiltinSymbols)
         messages = (EShMessages)(messages | EShMsgBuiltinSymbolTable);
+    if (EnhancedMsgs)
+        messages = (EShMessages)(messages | EShMsgEnhanced);
 }
 
 //
@@ -1301,6 +1306,9 @@ void CompileAndLinkShaderUnits(std::vector<ShaderCompUnit> compUnits)
         if (HlslDxPositionW)
             shader->setDxPositionW(true);
 
+        if (EnhancedMsgs)
+            shader->setEnhancedMsgs();
+
         // Set up the environment, some subsettings take precedence over earlier
         // ways of setting things.
         if (Options & EOptionSpv) {
@@ -1867,6 +1875,7 @@ void usage()
            "  --hlsl-dx-position-w              W component of SV_Position in HLSL fragment\n"
            "                                    shaders compatible with DirectX\n"
            "  --invert-y | --iy                 invert position.Y output in vertex shader\n"
+           "  --enhanced-msgs                   print more readable error messages (GLSL only)\n"
            "  --keep-uncalled | --ku            don't eliminate uncalled functions\n"
            "  --nan-clamp                       favor non-NaN operand in min, max, and clamp\n"
            "  --no-storage-format | --nsf       use Unknown image format\n"