Fix GCC 4 build after r364464

It was failing with:

In file included from /b/s/w/ir/cache/builder/src/third_party/llvm/llvm/lib/Bitcode/Reader/BitstreamReader.cpp:9:0:
/b/s/w/ir/cache/builder/src/third_party/llvm/llvm/include/llvm/Bitcode/BitstreamReader.h:
In member function 'llvm::Expected<long unsigned int> llvm::SimpleBitstreamCursor::ReadVBR64(unsigned int)':
/b/s/w/ir/cache/builder/src/third_party/llvm/llvm/include/llvm/Bitcode/BitstreamReader.h:262:14:
error: could not convert 'MaybeRead' from 'llvm::Expected<unsigned int>' to 'llvm::Expected<long unsigned int>'
       return MaybeRead;
              ^
/b/s/w/ir/cache/builder/src/third_party/llvm/llvm/include/llvm/Bitcode/BitstreamReader.h:279:16:
error: could not convert 'MaybeRead' from 'llvm::Expected<unsigned int>' to 'llvm::Expected<long unsigned int>'
         return MaybeRead;
                ^

llvm-svn: 364504