[Analysis] Migrate away from PointerUnion::dyn_cast (NFC) (#122652)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
because we expect Ctx->FunArgs to be nonnull.

Author: kazutakahirata

clang/lib/Analysis/ThreadSafetyCommon.cpp

@@ -336,7 +336,7 @@ til::SExpr *SExprBuilder::translateDeclRefExpr(const DeclRefExpr *DRE,
               : (cast<ObjCMethodDecl>(D)->getCanonicalDecl() == Canonical)) {
         // Substitute call arguments for references to function parameters
         if (const Expr *const *FunArgs =
-                Ctx->FunArgs.dyn_cast<const Expr *const *>()) {
+                dyn_cast<const Expr *const *>(Ctx->FunArgs)) {
           assert(I < Ctx->NumArgs);
           return translate(FunArgs[I], Ctx->Prev);
         }