[NFC][Clang] Fix potential dereferencing of nullptr (#86759)

This patch replaces dyn_cast<> with cast<> to resolve potential static
analyzer bugs for

1. Dereferencing a pointer issue with nullptr GVar when calling
addAttribute() in AIXTargetCodeGenInfo::setTargetAttributes(clang::Decl
const *, llvm::GlobalValue *, clang::CodeGen::CodeGenModule &).

2. Dereferencing a pointer issue with nullptr GG when calling
getCorrespondingConstructor() in
DeclareImplicitDeductionGuidesForTypeAlias(clang::Sema &,
clang::TypeAliasTemplateDecl *, clang::SourceLocation).

3. Dereferencing a pointer issue with nullptr CurrentBT when calling
getKind() in
ComplexExprEmitter::GetHigherPrecisionFPType(clang::QualType).