[lldb] Reactivate Objective-C++ plugin

Summary:
Since commit 7b3ef05a the Objective-C++ plugin is dead code.
That commit added Objective-C++ to the list of languages for which `Language::LanguageIsCPlusPlus`
returns true. As the C++ language plugin also uses that method to figure out if it is responsible for a
given language, the C++ plugin since then also became the plugin that we found when looking for
a language plugin for Objective-C++. The only real fallout from that is that the source highlighting
for Objective-C++ files never worked as we always found the C++ plugin which refuses to highlight
files with Objective-C++ extensions.

This patch just adds a special exception for Objective-C++ to the list of languages that are governed
by the C++ plugin. Also adds a test that makes sure that we find the right plugin for all C language
types and that the highlighting for `.mm` (Objective-C++) and `.m` (Objective-C) files works.

I didn't revert 7b3ef05a as it does make sense to return
true for Objective-C++ from `Language::LanguageIsCPlusPlus` (e.g., we currently check if we care about
ODR violations by doing `if (Language::LanguageIsCPlusPlus(...))` and this should also work for
Objective-C++).

Fixes rdar://64420183

Reviewers: aprantl

Reviewed By: aprantl

Subscribers: mgorny, abidh, JDevlieghere

Differential Revision: https://reviews.llvm.org/D82109