[NFC][DebugInfo] Fix Wreturn-type gcc warning

GCC warning:
```
/llvm-project/llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp: In member function ‘llvm::Expected<long unsigned int> llvm::dwarf::CFIProgram::Instruction::getOperandAsUnsigned(const llvm::dwarf::CFIProgram&, uint32_t) const’:
/llvm-project/llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp:425:1: warning: control reaches end of non-void function [-Wreturn-type]
  425 | }
      | ^
/llvm-project/llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp: In member function ‘llvm::Expected<long int> llvm::dwarf::CFIProgram::Instruction::getOperandAsSigned(const llvm::dwarf::CFIProgram&, uint32_t) const’:
/llvm-project/llvm/lib/DebugInfo/DWARF/DWARFDebugFrame.cpp:477:1: warning: control reaches end of non-void function [-Wreturn-type]
  477 | }
      | ^
```