[lldb] Make frame var --regex always search globals

Currently frame var --regex sometimes searches globals, sometimes it doesn't.
This happens because `StackFrame::GetVariableList` always returns the biggest
list it has, regardless of whether only globals were requested or not. In other
words, if a previous call to `GetVariableList` requested globals, all subsequent
calls will see them.

The implication here is that users of `StackFrame::GetVariableList` are expected
to filter the results of this function. This is what we do for a vanilla
`frame var` command. But it is not what we do when `--regex` is used. This
commit solves the issue by:

1. Making `--regex` imply `--globals`. This matches the behavior of `frame var
<some_name>`, which will also search the global scope.
2. Making the `--regex` search respect the command object options.

See the added test for an example of the oddities this patch addresses. Without
the patch, the test fails. However it could be made to pass by calling a plain
`frame var` before calling `frame var --regex A::`.

Differential Revision: https://reviews.llvm.org/D155334