[Bugfix] Avoid symbol conflicts in MakePackedAPI/MakeUnpackedAPI

PRs apache#14913 and
apache#14914 made analogous changes to
`MakePackedAPI` and `MakeUnpackedAPI` to handle subroutine calls.
Both PRs introduced the same symbol,
`tvm::tir::SubroutineCallRewriter`, a local utility to update internal
calls to a modified function.  While each PR passed CI individually,
and was therefore able to merge, having both changes caused a
duplicate symbol.

This commit updates `MakePackedAPI` and `MakeUnpackedAPI` to place
their local utilities into anonymous namespaces, avoiding the
conflict.

src/tir/transforms/make_packed_api.cc

@@ -42,6 +42,7 @@ namespace tir {
 
 static constexpr const char* kDeviceContextVar = "device_api_context";
 
+namespace {
 class ReturnRewriter : public StmtMutator {
  public:
   explicit ReturnRewriter(Var ret_var, Var ret_tcode) : ret_var_(ret_var), ret_tcode_(ret_tcode) {}
@@ -176,6 +177,8 @@ class SubroutineCallRewriter : public StmtExprMutator {
   bool made_change_{false};
 };
 
+}  // namespace
+
 inline Stmt MakeAssertEQ(PrimExpr lhs, PrimExpr rhs, std::string msg) {
   return AssertStmt(lhs == rhs, tvm::tir::StringImm(msg), Evaluate(0));
 }

src/tir/transforms/make_unpacked_api.cc

@@ -40,6 +40,8 @@
 namespace tvm {
 namespace tir {
 
+namespace {
+
 class SubroutineCallRewriter : public StmtExprMutator {
  public:
   static Optional<Stmt> Apply(const std::unordered_set<const GlobalVarNode*>& external_methods,
@@ -84,6 +86,8 @@ class SubroutineCallRewriter : public StmtExprMutator {
   bool made_change_{false};
 };
 
+}  // namespace
+
 PrimFunc MakeUnpackedAPI(PrimFunc func) {
   // A function with an explicit calling convention has already been
   // lowered, and should not be modified.