[ObjC] Encode pointers to C++ classes as "^v" if the encoded string

[ObjC] Encode pointers to C++ classes as "^v" if the encoded string
would otherwise include template specialization types

This helps reduce the size of the encoded C++ type strings in the binary.

This is enabled by default only on Darwin, but can be enabled/disabled
via command line options.

rdar://63288571

Differential Revision: https://reviews.llvm.org/D96816