[Clang] Prevent potential null pointer dereferences (llvm#117176)

This commit addresses several null pointer issues identified by static
analysis by replacing dyn_cast<> with cast<> and getAs<> with castAs<>
in various parts of the Clang codebase. The cast and castAs method is
used to ensure that the type is correctly cast, which helps prevent
potential null pointer dereferences.

Changes:
1. ASTContext.cpp:
Replaced dyn_cast with cast to ensure that the type is correctly cast to
AttributedType.

2. SemaFunctionEffects.cpp:
Replaced getAs with castAs to ensure that the type is correctly cast to
FunctionProtoType.

3. SemaHLSL.cpp:
Replaced getAs with castAs to ensure that the type is correctly cast to
VectorType.

Author: smanna12

clang/lib/AST/ASTContext.cpp

@@ -3558,7 +3558,7 @@ ASTContext::adjustType(QualType Orig,
                        llvm::function_ref<QualType(QualType)> Adjust) const {
   switch (Orig->getTypeClass()) {
   case Type::Attributed: {
-    const auto *AT = dyn_cast<AttributedType>(Orig);
+    const auto *AT = cast<AttributedType>(Orig);
     return getAttributedType(AT->getAttrKind(),
                              adjustType(AT->getModifiedType(), Adjust),
                              adjustType(AT->getEquivalentType(), Adjust),

clang/lib/Sema/SemaFunctionEffects.cpp

@@ -627,7 +627,7 @@ class Analyzer {
           IsNoexcept = isNoexcept(FD);
         } else if (auto *BD = dyn_cast<BlockDecl>(D)) {
           if (auto *TSI = BD->getSignatureAsWritten()) {
-            auto *FPT = TSI->getType()->getAs<FunctionProtoType>();
+            auto *FPT = TSI->getType()->castAs<FunctionProtoType>();
             IsNoexcept = FPT->isNothrow() || BD->hasAttr<NoThrowAttr>();
           }
         }

clang/lib/Sema/SemaHLSL.cpp

@@ -1908,9 +1908,9 @@ bool SemaHLSL::CheckBuiltinFunctionCall(unsigned BuiltinID, CallExpr *TheCall) {
       return true;
     // ensure both args have 3 elements
     int NumElementsArg1 =
-        TheCall->getArg(0)->getType()->getAs<VectorType>()->getNumElements();
+        TheCall->getArg(0)->getType()->castAs<VectorType>()->getNumElements();
     int NumElementsArg2 =
-        TheCall->getArg(1)->getType()->getAs<VectorType>()->getNumElements();
+        TheCall->getArg(1)->getType()->castAs<VectorType>()->getNumElements();
 
     if (NumElementsArg1 != 3) {
       int LessOrMore = NumElementsArg1 > 3 ? 1 : 0;