[lldb] Remove CPlusPlusLanguage from Mangled

The only remaining plugin dependency in Mangled is CPlusPlusLanguage which it
uses to extract information from C++ mangled names. The static function
GetDemangledNameWithoutArguments is written specifically for C++, so it
would make sense for this specific functionality to live in a
C++-related plugin. In order to keep this functionality in Mangled
without maintaining this dependency, I added
`Language::GetDemangledFunctionNameWithoutArguments`.

Differential Revision: https://reviews.llvm.org/D105215