[AST] Migrate away from PointerUnion::dyn_cast (NFC) (#123156)

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 IntegerType to be nonnull.

Author: kazutakahirata

clang/include/clang/AST/Decl.h

@@ -4021,7 +4021,7 @@ class EnumDecl : public TagDecl {
   QualType getIntegerType() const {
     if (!IntegerType)
       return QualType();
-    if (const Type *T = IntegerType.dyn_cast<const Type*>())
+    if (const Type *T = dyn_cast<const Type *>(IntegerType))
       return QualType(T, 0);
     return cast<TypeSourceInfo *>(IntegerType)->getType().getUnqualifiedType();
   }