[libc++] fix `shared_ptr`'s incorrect constraints

Fix several bugs:
1. https://llvm.org/PR60258
   The conversion constructors' constraint `__compatible_with` incorrectly allow array types conversion to scalar types
2. https://llvm.org/PR53368
   The constructor that takes `unique_ptr` are not suffiently constrained.
3. The constructors that take raw pointers incorretly use `__compatible_with`. They have different constraints

Differential Revision: https://reviews.llvm.org/D143346