Set the HLSL supported compiler version to 2018 (#6502)

Co-authored-by: Jay Kwak <82421531+jkwak-work@users.noreply.github.com>

Author: zlatinski

source/slang/slang.cpp

@@ -2365,7 +2365,7 @@ Dictionary<String, String> TranslationUnitRequest::getCombinedPreprocessorDefini
     // variables.
     {
         // Used to identify level of HLSL language compatibility
-        combinedPreprocessorDefinitions.addIfNotExists("__HLSL_VERSION", "2020");
+        combinedPreprocessorDefinitions.addIfNotExists("__HLSL_VERSION", "2018");
 
         // Indicates this is being compiled by the slang *compiler*
         combinedPreprocessorDefinitions.addIfNotExists("__SLANG_COMPILER__", "1");

tests/preprocessor/simple-pre-defined.hlsl.expected

@@ -2,5 +2,5 @@ result code = 0
 standard error = {
 }
 standard output = {
-1 2020 0 1 
+1 2018 0 1 
 }

tests/preprocessor/simple-pre-defined.slang.expected

@@ -2,5 +2,5 @@ result code = 0
 standard error = {
 }
 standard output = {
-1 2020 1 0 
+1 2018 1 0 
 }