[CodeGen] Migrate away from PointerUnion::dyn_cast (NFC) (#123013)

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 Data to be nonnull.

Author: kazutakahirata

clang/lib/CodeGen/CGOpenMPRuntime.cpp

@@ -7768,7 +7768,7 @@ class MappableExprsHandler {
              &Data : RecordLayout) {
       if (Data.isNull())
         continue;
-      if (const auto *Base = Data.dyn_cast<const CXXRecordDecl *>())
+      if (const auto *Base = dyn_cast<const CXXRecordDecl *>(Data))
         getPlainLayout(Base, Layout, /*AsBase=*/true);
       else
         Layout.push_back(cast<const FieldDecl *>(Data));