[Symtab][NFC] Added llvm_unreachable to supress compiler warning

[JDevlieghere]

Looks reasonable. Did you run into this or is it just an improvement as you came across the code?

[xbolva00]

In D51587#1222009, @JDevlieghere wrote:

Looks reasonable. Did you run into this or is it just an improvement as you came across the code?

I just saw it as one of many GCC warnings when building lldb trunk.

[JDevlieghere]

Alright, thanks for the fix!

source/Symbol/Symtab.cpp

static bool lldb_skip_name(llvm::StringRef mangled,
   // No filters for this scheme yet. Include all names in indexing.
   case Mangled::eManglingSchemeMSVC:
     return false;
 
   // Don't try and demangle things we can't categorize.
   case Mangled::eManglingSchemeNone:
     return true;
   }
+  llvm_unreachable("unknown scheme!");
 }
 
 void Symtab::InitNameIndexes() {
   // Protected function, no need to lock mutex...
   if (!m_name_indexes_computed) {
     m_name_indexes_computed = true;
     static Timer::Category func_cat(LLVM_PRETTY_FUNCTION);
     Timer scoped_timer(func_cat, "%s", LLVM_PRETTY_FUNCTION);