[llvm-cov] Fix -Wcovered-switch-default in CoverageMapping.cpp (NFC)

/Users/jiefu/llvm-project/llvm/lib/ProfileData/Coverage/CoverageMapping.cpp:959:3: error: default label in switch which covers all enumeration values [-Werror,-Wcovered-switch-default]
  default:
  ^
1 error generated.