[lldb] Add unreachable after fully covered switches, avoid GCC warnings. NFC. (#159327)

This avoids the following kind of warning with GCC:

    warning: control reaches end of non-void function [-Wreturn-type]

Author: mstorsjo

lldb/source/Plugins/Language/CPlusPlus/CPlusPlusLanguage.cpp

@@ -2199,6 +2199,7 @@ bool CPlusPlusLanguage::GetFunctionDisplayName(
   case FunctionNameRepresentation::eName:
     return false;
   }
+  llvm_unreachable("Fully covered switch above");
 }
 
 bool CPlusPlusLanguage::HandleFrameFormatVariable(

lldb/source/Plugins/Process/Utility/HistoryUnwind.cpp

@@ -60,6 +60,7 @@ static bool BehavesLikeZerothFrame(HistoryPCType pc_type, uint32_t frame_idx) {
   case HistoryPCType::Calls:
     return true;
   }
+  llvm_unreachable("Fully covered switch above");
 }
 
 bool HistoryUnwind::DoGetFrameInfoAtIndex(uint32_t frame_idx, lldb::addr_t &cfa,

lldb/source/Plugins/Process/wasm/ProcessWasm.cpp

@@ -98,6 +98,7 @@ size_t ProcessWasm::ReadMemory(lldb::addr_t vm_addr, void *buf, size_t size,
         "Wasm read failed for invalid address 0x%" PRIx64, vm_addr);
     return 0;
   }
+  llvm_unreachable("Fully covered switch above");
 }
 
 llvm::Expected<std::vector<lldb::addr_t>>

lldb/source/Plugins/SymbolFile/DWARF/SymbolFileDWARF.cpp

@@ -2502,6 +2502,7 @@ static llvm::StringRef ClangToItaniumCtorKind(clang::CXXCtorType kind) {
   case clang::CXXCtorType::Ctor_Comdat:
     llvm_unreachable("Unexpected constructor kind.");
   }
+  llvm_unreachable("Fully covered switch above");
 }
 
 static llvm::StringRef ClangToItaniumDtorKind(clang::CXXDtorType kind) {
@@ -2517,6 +2518,7 @@ static llvm::StringRef ClangToItaniumDtorKind(clang::CXXDtorType kind) {
   case clang::CXXDtorType::Dtor_Comdat:
     llvm_unreachable("Unexpected destructor kind.");
   }
+  llvm_unreachable("Fully covered switch above");
 }
 
 static llvm::StringRef