[lldb] Reject redefinitions of persistent variables

Currently one can redefine a persistent variable and LLDB will just silently
ignore the second definition:

```
(lldb) expr int $i = 1
(lldb) expr int $i = 2
(lldb) expr $i
(int) $i = 1
```

This patch makes this an error and rejects the expression with the second
definition.

A nice follow up would be to refactor LLDB's persistent variables to not just be
a pair of type and name, but also contain some way to obtain the original
declaration and source code that declared the variable. That way we could
actually make a full diagnostic as we would get from redefining a variable twice
in the same expression.

Reviewed By: labath, shafik, JDevlieghere

Differential Revision: https://reviews.llvm.org/D89310