[libc++][PMR] Move the pmr::memory_resource destructor into the dylib

This avoids emitting the VTable of `pmr::memory_resource` in every TU.

Reviewed By: ldionne

Spies: EricWF, nemanjai, libcxx-commits

Differential Revision: https://reviews.llvm.org/D136196
diff --git a/libcxx/src/memory_resource.cpp b/libcxx/src/memory_resource.cpp
index 571a3a0..d4a735b 100644
--- a/libcxx/src/memory_resource.cpp
+++ b/libcxx/src/memory_resource.cpp
@@ -24,7 +24,7 @@
 
 // memory_resource
 
-//memory_resource::~memory_resource() {}
+memory_resource::~memory_resource() = default;
 
 // new_delete_resource()