[lldb] Move redundant persistent variable counter to ClangPersistentVariables

Currently Target::m_next_persistent_variable_index is counting up
for our persistent variables ($0, $1, ...) but we also have a
unused counter that is supposed to do this in
ClangPersistentVariables but that stays always at 0 (because
we currently increase the target counter when we should increase
that unused counter).

This patch removes the counter in Target and lets the documented
counter in ClangPersistentVariables do the variable counting.

Patch *should* be NFC, but it might unexpectedly bring LLDB to
new code paths that could contain exciting new bugs to fix.

llvm-svn: 367842