[SYCL][NFC] Avoid nullptr dereferencing of YDimExpr and ZDimExpr (#4367)

Add assert that arguments of work_group_size attributes cannot be nullptr

Klocwork exposed a (false positive) bug that nullptr derferencing of 
YDimExpr and ZDimExpr is possible for non sycl:: usage.
This is not practically possible because such (OpenCL and cl::) spellings
of the attribute require three arguments.

This patch
1.  Removes the ability of SetDefaultValue lambda to return nullptr
2. Asserts in SetDefaultValue that it is not possible for sycl:: case 
to have NULL arguments
3.  Moves error checking for three arguments for OpenCL and cl:: cases 
ahead of the assert 
4. Removes check for intel::reqd_work_group_size spelling

Author: schittir

clang/lib/Sema/SemaDeclAttr.cpp

@@ -3141,29 +3141,6 @@ static void handleWorkGroupSize(Sema &S, Decl *D, const ParsedAttr &AL) {
     return;
 
   S.CheckDeprecatedSYCLAttributeSpelling(AL);
-
-  Expr *XDimExpr = AL.getArgAsExpr(0);
-
-  // If no attribute argument is specified, set the second and third argument
-  // to the default value 1, but only if the sycl:: or intel::
-  // reqd_work_group_size spelling was used.
-  auto SetDefaultValue = [](Sema &S, const ParsedAttr &AL, SourceLocation loc) {
-    Expr *E =
-        (AL.getKind() == ParsedAttr::AT_ReqdWorkGroupSize && AL.hasScope() &&
-         (AL.getScopeName()->isStr("sycl") ||
-          AL.getScopeName()->isStr("intel")))
-            ? IntegerLiteral::Create(S.Context, llvm::APInt(32, 1),
-                                     S.Context.IntTy, AL.getLoc())
-            : nullptr;
-    return E;
-  };
-
-  Expr *YDimExpr = AL.isArgExpr(1) ? AL.getArgAsExpr(1)
-                                   : SetDefaultValue(S, AL, AL.getLoc());
-
-  Expr *ZDimExpr = AL.isArgExpr(2) ? AL.getArgAsExpr(2)
-                                   : SetDefaultValue(S, AL, AL.getLoc());
-
   // __attribute__((reqd_work_group_size)), [[cl::reqd_work_group_size]], and
   // [[intel::max_work_group_size]] all require exactly three arguments.
   if ((AL.getKind() == ParsedAttr::AT_ReqdWorkGroupSize &&
@@ -3175,6 +3152,24 @@ static void handleWorkGroupSize(Sema &S, Decl *D, const ParsedAttr &AL) {
       return;
   }
 
+  Expr *XDimExpr = AL.getArgAsExpr(0);
+
+  // If no attribute argument is specified, set the second and third argument
+  // to the default value 1, but only if the sycl::reqd_work_group_size
+  // spelling was used.
+  auto SetDefaultValue = [](Sema &S, const ParsedAttr &AL) {
+    assert(AL.getKind() == ParsedAttr::AT_ReqdWorkGroupSize && AL.hasScope() &&
+           AL.getScopeName()->isStr("sycl"));
+    return IntegerLiteral::Create(S.Context, llvm::APInt(32, 1),
+                                  S.Context.IntTy, AL.getLoc());
+  };
+
+  Expr *YDimExpr =
+      AL.isArgExpr(1) ? AL.getArgAsExpr(1) : SetDefaultValue(S, AL);
+
+  Expr *ZDimExpr =
+      AL.isArgExpr(2) ? AL.getArgAsExpr(2) : SetDefaultValue(S, AL);
+
   ASTContext &Ctx = S.getASTContext();
 
   if (!XDimExpr->isValueDependent() && !YDimExpr->isValueDependent() &&