[llvm] [MC] Rename PrivateGlobalPrefix to InternalSymbolPrefix. NFC (PR #185164)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> What is the naming plan for PrivateLabelPrefix and LinkerPrivateGlobalPrefix?

PrivateLabelPrefix will be removed.

For all targets except BPF, PrivateLabelPrefix matches PrivateGlobalPrefix. The BPF difference was an oversight instead of a deliberate design choice. Waiting on your https://github.com/llvm/llvm-project/pull/95103 :)

> I always interpreted PrivateGlobalPrefix as the prefix for globals that have `private` linkage in LLVM IR and from that PoV, this change is replacing one inconsistency with another.

Agreed. However, the object file format uses dominate while the llvm/lib/IR/Mangler.cpp centric PrivateGlobalPrefix is quite misplaced. 

`LinkerPrivateGlobalPrefix` is a Mach-O feature. We could rename it to `LinkerPrivateSymbolPrefix`.

https://github.com/llvm/llvm-project/pull/185164