[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