[DWARF][M68k] Add new DW_CC for the new M68kRTD calling convention

[myhsu]

Summary

Add DW_CC_LLVM_M68kRTD to model the new llvm::CallingConv::M68kRTD (see D149864 and D149867).

To my best understanding since DW_CC_LLVM_* are considered vendor extensions, we can just add it to LLVM without being ratified or even adding into GCC/GDB. Please let me know if I'm wrong.

[aprantl]

LGTM, is it easy to add a test that runs llvm-dwarfdump on some yaml2obj or llc output?

[myhsu]

Add a llc test to check if DW_CC_LLVM_M68kRTD is printed correctly in llvm-dwarfdump.

llvm/include/llvm/BinaryFormat/Dwarf.def

 HANDLE_DW_CC(0xc4, LLVM_AAPCS_VFP)
 HANDLE_DW_CC(0xc5, LLVM_IntelOclBicc)
 HANDLE_DW_CC(0xc6, LLVM_SpirFunction)
 HANDLE_DW_CC(0xc7, LLVM_OpenCLKernel)
 HANDLE_DW_CC(0xc8, LLVM_Swift)
 HANDLE_DW_CC(0xc9, LLVM_PreserveMost)
 HANDLE_DW_CC(0xca, LLVM_PreserveAll)
 HANDLE_DW_CC(0xcb, LLVM_X86RegCall)
+HANDLE_DW_CC(0xcc, LLVM_M68kRTD)
 // From GCC source code (include/dwarf2.h): This DW_CC_ value is not currently
 // generated by any toolchain.  It is used internally to GDB to indicate OpenCL
 // C functions that have been compiled with the IBM XL C for OpenCL compiler and
 // use a non-platform calling convention for passing OpenCL C vector types.
 HANDLE_DW_CC(0xff, GDB_IBM_OpenCL)
 
 // Line Number Extended Opcode Encodings
 HANDLE_DW_LNE(0x01, end_sequence)

llvm/lib/DebugInfo/DWARF/DWARFTypePrinter.cpp

case CallingConvention::DW_CC_LLVM_PreserveMost:
       OS << " __attribute__((preserve_most))";
       break;
     case CallingConvention::DW_CC_LLVM_PreserveAll:
       OS << " __attribute__((preserve_all))";
       break;
     case CallingConvention::DW_CC_LLVM_X86RegCall:
       OS << " __attribute__((regcall))";
       break;
+    case CallingConvention::DW_CC_LLVM_M68kRTD:
+      OS << " __attribute__((m68k_rtd))";
+      break;
     }
   }
 
   if (Const)
     OS << " const";
   if (Volatile)
     OS << " volatile";
   if (D.find(DW_AT_reference))