ARMDefines.h: fix -Wimplicit-fallthrough in -DLLVM_ENABLE_ASSERTIONS=OFF builds

[MaskRay]

I've seen these warnings for a very long time...

[677/732] Building CXX object tools/lldb/source/API/CMakeFiles/liblldb.dir/SystemInitializerFull.cpp.o          
In file included from ../tools/lldb/source/API/SystemInitializerFull.cpp:46:                                   
In file included from ../tools/lldb/source/./Plugins/Instruction/ARM/EmulateInstructionARM.h:12:               
../tools/lldb/source/./Plugins/Process/Utility/ARMDefines.h:73:3: warning: unannotated fall-through between swit
ch labels [-Wimplicit-fallthrough]
  case COND_EQ:
  ^
../tools/lldb/source/./Plugins/Process/Utility/ARMDefines.h:73:3: note: insert 'LLVM_FALLTHROUGH;' to silence th
is warning
  case COND_EQ:
  ^
  LLVM_FALLTHROUGH; 
../tools/lldb/source/./Plugins/Process/Utility/ARMDefines.h:73:3: note: insert 'break;' to avoid fall-through  
  case COND_EQ:
  ^
  break; 
1 warning generated.
[732/732] Linking CXX executable bin/lldb

[rovka]

Why not remove the default clause and put the unreachable after the switch?

[JDevlieghere]

In D61844#1499576, @rovka wrote:

Why not remove the default clause and put the unreachable after the switch?

+1

[MaskRay]

Move llvm_unreachable after the switch

[JDevlieghere]

LGTM

source/Plugins/Process/Utility/ARMDefines.h

 //===-- ARMDefines.h --------------------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef lldb_ARMDefines_h_
 #define lldb_ARMDefines_h_
 
+#include "llvm/Support/ErrorHandling.h"
+
 #include <cassert>
 #include <cstdint>
 
 // Common definitions for the ARM/Thumb Instruction Set Architecture.
 
 namespace lldb_private {
 
 // ARM shifter types
#define COND_LE                                                                \
   0xD // Less than or equal        <, ==, or unordered           Z == 1 or N !=
       // V
 #define COND_AL                                                                \
   0xE // Always (unconditional)    Always (unconditional)        Any
 #define COND_UNCOND 0xF
 
 static inline const char *ARMCondCodeToString(uint32_t CC) {
   switch (CC) {
-  default:
-    assert(0 && "Unknown condition code");
   case COND_EQ:
     return "eq";
   case COND_NE:
     return "ne";
   case COND_HS:
     return "hs";
   case COND_LO:
     return "lo";
case COND_LT:
     return "lt";
   case COND_GT:
     return "gt";
   case COND_LE:
     return "le";
   case COND_AL:
     return "al";
   }
+  llvm_unreachable("Unknown condition code");
 }
 
 static inline bool ARMConditionPassed(const uint32_t condition,
                                       const uint32_t cpsr) {
   const uint32_t cpsr_n = (cpsr >> 31) & 1u; // Negative condition code flag
   const uint32_t cpsr_z = (cpsr >> 30) & 1u; // Zero condition code flag
   const uint32_t cpsr_c = (cpsr >> 29) & 1u; // Carry condition code flag
   const uint32_t cpsr_v = (cpsr >> 28) & 1u; // Overflow condition code flag