[llvm-readobj] - An attempt to fix BB.

http://lab.llvm.org:8011/builders/clang-cmake-x86_64-avx2-linux/builds/15710/steps/build%20stage%201/logs/stdio
fails with:

/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp: In function ‘llvm::Expected<const llvm::object::Elf_Mips_Options<ELFT>*> readMipsOptions(const uint8_t*, llvm::ArrayRef<unsigned char>&, bool&)’:
/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp:3373:19: error: the value of ‘ExpectedSize’ is not usable in a constant expression
     if (O->size < ExpectedSize)
                   ^
/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp:3369:10: note: ‘size_t ExpectedSize’ is not const
   size_t ExpectedSize =
          ^
/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp:3373:12: error: parse error in template argument list
     if (O->size < ExpectedSize)
            ^
/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp: In instantiation of ‘llvm::Expected<const llvm::object::Elf_Mips_Options<ELFT>*> readMipsOptions(const uint8_t*, llvm::ArrayRef<unsigned char>&, bool&) [with ELFT = llvm::object::ELFType<(llvm::support::endianness)0u, true>; uint8_t = unsigned char]’:
/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp:3400:30:   required from ‘void {anonymous}::ELFDumper<ELFT>::printMipsOptions() [with ELFT = llvm::object::ELFType<(llvm::support::endianness)0u, true>]’
/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp:2878:21:   required from ‘void {anonymous}::ELFDumper<ELFT>::printArchSpecificInfo() [with ELFT = llvm::object::ELFType<(llvm::support::endianness)0u, true>]’
/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp:6999:1:   required from here
/home/ssglocal/clang-cmake-x86_64-avx2-linux/clang-cmake-x86_64-avx2-linux/llvm/llvm/tools/llvm-readobj/ELFDumper.cpp:3373:5: error: ‘size’ is not a member template function

Fix: add 2 `const` words to variables.