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.

rdar://problem/39299889

Differential Revision: https://reviews.llvm.org/D46083

llvm-svn: 331234