[AArch64] Fix a warning

This patch fixes:

  llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:7157:3: error:
  unannotated fall-through between switch labels
  [-Werror,-Wimplicit-fallthrough]

Author: kazutakahirata

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -7154,6 +7154,7 @@ SDValue AArch64TargetLowering::LowerINIT_TRAMPOLINE(SDValue Op,
   switch (CC) {
   default:
     NestReg = 0x0f; // X15
+    LLVM_FALLTHROUGH;
   case CallingConv::ARM64EC_Thunk_Native:
   case CallingConv::ARM64EC_Thunk_X64:
     // Must be kept in sync with AArch64CallingConv.td