[lldb] Fix that running a top level expression without a process fails with a cryptic error

Right now when running `expr --top-level -- void foo() {}`, LLDB just prints a cryptic
`error: Couldn't find $__lldb_expr() in the module` error. The reason for that is
that if we don't have a running process, we try to set our execution policy to always use the
IR interpreter (ExecutionPolicyNever) which works even without a process. However
that code didn't consider the special ExecutionPolicyTopLevel which we use for
top-level expressions. By changing the execution policy to ExecutionPolicyNever,
LLDB thinks we're actually trying to interpret a normal expression inside our
`$__lldb_expr` function and then fails when looking for it.

This just adds an exception for top-level expressions to that code and a bunch of tests.

Reviewed By: shafik

Differential Revision: https://reviews.llvm.org/D91723