Fix -Wunused-variable warnings after r364464

/work/llvm.monorepo/llvm/lib/Bitcode/Reader/BitcodeReader.cpp: In function
‘llvm::Expected<std::basic_string<char> > readIdentificationBlock(llvm::BitstreamCursor&)’:
/work/llvm.monorepo/llvm/lib/Bitcode/Reader/BitcodeReader.cpp:205:22:
warning: unused variable ‘BitCode’ [-Wunused-variable]
     switch (unsigned BitCode = MaybeBitCode.get()) {
                      ^
/work/llvm.monorepo/llvm/lib/Bitcode/Reader/BitcodeReader.cpp: In member function
‘llvm::Error {anonymous}::ModuleSummaryIndexBitcodeReader::parseModule()’:
/work/llvm.monorepo/llvm/lib/Bitcode/Reader/BitcodeReader.cpp:5367:26:
warning: unused variable ‘BitCode’ [-Wunused-variable]
         switch (unsigned BitCode = MaybeBitCode.get()) {
                          ^

llvm-svn: 364505