[lldb] Add llvm_unreachable in RichManglingContext

[kastiglione]

Summary

Add llvm_unreachable to prevent warnings/errors in gcc and msvc.

[JDevlieghere]

LGTM

lldb/source/Core/RichManglingContext.cpp

case ItaniumPartialDemangler: {
     return processIPDStrResult(buf, n);
   }
   case PluginCxxLanguage:
     return get<CPlusPlusLanguage::MethodName>(m_cxx_method_parser)
         ->GetBasename();
   case None:
     return {};
   }
+  llvm_unreachable("Fully covered switch above!");
 }
 
 llvm::StringRef RichManglingContext::ParseFunctionDeclContextName() {
   assert(m_provider != None && "Initialize a provider first");
   switch (m_provider) {
   case ItaniumPartialDemangler: {
     auto n = m_ipd_buf_size;
     auto buf = m_ipd.getFunctionDeclContextName(m_ipd_buf, &n);
     return processIPDStrResult(buf, n);
   }
   case PluginCxxLanguage:
     return get<CPlusPlusLanguage::MethodName>(m_cxx_method_parser)
         ->GetContext();
   case None:
     return {};
   }
+  llvm_unreachable("Fully covered switch above!");
 }
 
 llvm::StringRef RichManglingContext::ParseFullName() {
   assert(m_provider != None && "Initialize a provider first");
   switch (m_provider) {
   case ItaniumPartialDemangler: {
     auto n = m_ipd_buf_size;
     auto buf = m_ipd.finishDemangle(m_ipd_buf, &n);
     return processIPDStrResult(buf, n);
   }
   case PluginCxxLanguage:
     return get<CPlusPlusLanguage::MethodName>(m_cxx_method_parser)
         ->GetFullName()
         .GetStringRef();
   case None:
     return {};
   }
+  llvm_unreachable("Fully covered switch above!");
 }