[lldb] Remove Expression's dependency on CPlusPlusLanguagePlugin

This change accomplishes the following:
- Moves `IRExecutionUnit::FindBestAlternateMangledName` to `Language`.
- Renames `FindBestAlternateMangledName` to
  `FindBestAlternateFunctionMangledName`
- Changes the first parameter of said method from a `ConstString`
  representing a demangled name to a `Mangled`.
- Remove the use of CPlusPlusLanguage from Expression