This patch attempts to decouple C++ expression evaluation from
Objective-C support. We've previously enabled it by default (if a
runtime existed), but that meant we're opting into extra work we only
need to do for Objective-C, which complicates/slows down C++ expression
evaluation. Of course there's a valid use-case for this, which is
calling Objective-C APIs when stopped in C++ frames (which Objective-C++
developers might want to do). In those cases we should really prompt the
user to add the `expr --language objc++` flag. To accomodate a likely
frequent use-case where a user breaks in a system C++ library (without
debug-symbols) but their application is actually an Objective-C app, we
allow Objective-C support in C++ expressions if the current frame
doesn't have debug-info.

This fixes https://github.com/llvm/llvm-project/issues/75443 and allows
us to add more `LangOpts.ObjC` guards around the expression evaluator in
the future (e.g., we could avoid looking into the Objective-C runtime
during C++ expression evaluation, which we currently do
unconditionally).

Depends on https://github.com/llvm/llvm-project/pull/87657