[lldb] Silence GCC warnings about missing returns after fully covered switches. NFC.

This silences warnings like this:

lldb/source/Core/DebuggerEvents.cpp: In member function ‘llvm::StringRef lldb_private::DiagnosticEventData::GetPrefix() const’:
lldb/source/Core/DebuggerEvents.cpp:55:1: warning: control reaches end of non-void function [-Wreturn-type]
   55 | }

Differential Revision: https://reviews.llvm.org/D123203

lldb/source/Breakpoint/Breakpoint.cpp

@@ -1026,6 +1026,7 @@ const char *Breakpoint::BreakpointEventTypeAsCString(BreakpointEventType type) {
     case eBreakpointEventTypeThreadChanged: return "thread changed";
     case eBreakpointEventTypeAutoContinueChanged: return "autocontinue changed";
   };
+  llvm_unreachable("Fully covered switch above!");
 }
 
 Log *Breakpoint::BreakpointEventData::GetLogChannel() {

lldb/source/Core/DebuggerEvents.cpp

@@ -52,6 +52,7 @@ llvm::StringRef DiagnosticEventData::GetPrefix() const {
   case Type::Error:
     return "error";
   }
+  llvm_unreachable("Fully covered switch above!");
 }
 
 void DiagnosticEventData::Dump(Stream *s) const {

lldb/source/Plugins/Process/Linux/IntelPTCollector.cpp

@@ -102,6 +102,7 @@ static Expected<uint32_t> ReadIntelPTConfigFile(const char *file,
     case BitOffset:
       return 10;
     }
+    llvm_unreachable("Fully covered switch above!");
   };
 
   auto createError = [&](const char *expected_value_message) {