Move the persistent variable counter into Target
so it can be shared across multiple language plugins.
In a multi-language project it is counterintuitive to have a result
variables reuse numbers just because they are using a different
language plugin in LLDB (but not for example, when they are
Objective-C versus C++, since they are both handled by Clang).
This is NFC on llvm.org except for the Go plugin.
Differential Revision: https://reviews.llvm.org/D46083