[CodeGen] Migrate away from PointerUnion::dyn_cast (NFC) (#124076)

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 Pos to be nonnull.

Author: kazutakahirata

clang/lib/CodeGen/CGOpenMPRuntime.cpp

@@ -4079,7 +4079,7 @@ static void emitDependData(CodeGenFunction &CGF, QualType &KmpDependInfoTy,
       Size = llvm::ConstantInt::get(CGF.SizeTy, 0);
     }
     LValue Base;
-    if (unsigned *P = Pos.dyn_cast<unsigned *>()) {
+    if (unsigned *P = dyn_cast<unsigned *>(Pos)) {
       Base = CGF.MakeAddrLValue(
           CGF.Builder.CreateConstGEP(DependenciesArray, *P), KmpDependInfoTy);
     } else {
@@ -4109,7 +4109,7 @@ static void emitDependData(CodeGenFunction &CGF, QualType &KmpDependInfoTy,
     CGF.EmitStoreOfScalar(
         llvm::ConstantInt::get(LLVMFlagsTy, static_cast<unsigned int>(DepKind)),
         FlagsLVal);
-    if (unsigned *P = Pos.dyn_cast<unsigned *>()) {
+    if (unsigned *P = dyn_cast<unsigned *>(Pos)) {
       ++(*P);
     } else {
       LValue &PosLVal = *cast<LValue *>(Pos);