[NFC][Clang] Fix potential dereferencing of nullptr (#85944)

This patch replaces getAs<> with castAs<> to resolve potential static
analyzer bugs for

1. Dereferencing a pointer issue with nullptr FPT when calling
ResolveExceptionSpec() in
    checkEscapingByref(clang::VarDecl *, clang::Sema &).

3. Dereferencing a pointer issue with nullptr ElementTy->getAs() when
calling getElementType() in
clang::Sema::SemaBuiltinFPClassification(clang::CallExpr *, unsigned
int).

4. Dereferencing a pointer issue with nullptr ConvType->getAs() when
calling getKeyword() in
clang::Sema::ActOnConversionDeclarator(clang::CXXConversionDecl *).

clang/lib/Sema/Sema.cpp

@@ -2207,7 +2207,7 @@ static void checkEscapingByref(VarDecl *VD, Sema &S) {
   // block copy/destroy functions. Resolve it here.
   if (const CXXRecordDecl *RD = T->getAsCXXRecordDecl())
     if (CXXDestructorDecl *DD = RD->getDestructor()) {
-      auto *FPT = DD->getType()->getAs<FunctionProtoType>();
+      auto *FPT = DD->getType()->castAs<FunctionProtoType>();
       S.ResolveExceptionSpec(Loc, FPT);
     }
 }

clang/lib/Sema/SemaChecking.cpp

@@ -9709,7 +9709,7 @@ bool Sema::SemaBuiltinFPClassification(CallExpr *TheCall, unsigned NumArgs,
   // vector argument can be supported in all of them.
   if (ElementTy->isVectorType() && IsFPClass) {
     VectorResultTy = GetSignedVectorType(ElementTy);
-    ElementTy = ElementTy->getAs<VectorType>()->getElementType();
+    ElementTy = ElementTy->castAs<VectorType>()->getElementType();
   }
 
   // This operation requires a non-_Complex floating-point number.

clang/lib/Sema/SemaDeclCXX.cpp

@@ -11353,7 +11353,7 @@ Decl *Sema::ActOnConversionDeclarator(CXXConversionDecl *Conversion) {
     if (ConvType->isUndeducedAutoType()) {
       Diag(Conversion->getTypeSpecStartLoc(), diag::err_auto_not_allowed)
           << getReturnTypeLoc(Conversion).getSourceRange()
-          << llvm::to_underlying(ConvType->getAs<AutoType>()->getKeyword())
+          << llvm::to_underlying(ConvType->castAs<AutoType>()->getKeyword())
           << /* in declaration of conversion function template= */ 24;
     }