[Sema] Migrate away from PointerUnion::dyn_cast (NFC) (#123014)

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 EnumUnderlying to be nonnull.

Author: kazutakahirata

clang/lib/Sema/SemaDecl.cpp

@@ -17310,7 +17310,7 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
         return nullptr;
       if (EnumUnderlying) {
         EnumDecl *ED = cast<EnumDecl>(New);
-        if (TypeSourceInfo *TI = EnumUnderlying.dyn_cast<TypeSourceInfo *>())
+        if (TypeSourceInfo *TI = dyn_cast<TypeSourceInfo *>(EnumUnderlying))
           ED->setIntegerTypeSourceInfo(TI);
         else
           ED->setIntegerType(QualType(cast<const Type *>(EnumUnderlying), 0));
@@ -17943,7 +17943,7 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
 
     if (EnumUnderlying) {
       EnumDecl *ED = cast<EnumDecl>(New);
-      if (TypeSourceInfo *TI = EnumUnderlying.dyn_cast<TypeSourceInfo*>())
+      if (TypeSourceInfo *TI = dyn_cast<TypeSourceInfo *>(EnumUnderlying))
         ED->setIntegerTypeSourceInfo(TI);
       else
         ED->setIntegerType(QualType(cast<const Type *>(EnumUnderlying), 0));