With latest llvm/clang, we have the following compilation errors:

    /home/yhs/work/bcc2/src/cc/frontends/clang/b_frontend_action.cc:
      In member function ‘bool ebpf::BTypeVisitor::VisitVarDecl(clang::VarDecl*)’:
    /home/yhs/work/bcc2/src/cc/frontends/clang/b_frontend_action.cc:1130:52:
      error: no matching function for call to ‘clang::Expr::EvaluateAsInt(llvm::APSInt&, clang::ASTContext&)’
               if (I->getInit(idx)->EvaluateAsInt(res, C)) {
                                                    ^
    /home/yhs/work/bcc2/src/cc/frontends/clang/b_frontend_action.cc:1130:52: note: candidate is:
    In file included from /home/yhs/work/llvm/build/install/include/clang/AST/Attr.h:19:0,
                 from /home/yhs/work/llvm/build/install/include/clang/AST/TypeLoc.h:18,
                 from /home/yhs/work/llvm/build/install/include/clang/AST/ASTTypeTraits.h:24,
                 from /home/yhs/work/llvm/build/install/include/clang/AST/ASTContext.h:18,
                 from /home/yhs/work/bcc2/src/cc/frontends/clang/b_frontend_action.cc:23:
    /home/yhs/work/llvm/build/install/include/clang/AST/Expr.h:604:8:
      note: bool clang::Expr::EvaluateAsInt(clang::Expr::EvalResult&, const clang::ASTContext&,
              clang::Expr::SideEffectsKind) const
       bool EvaluateAsInt(EvalResult &Result, const ASTContext &Ctx,
        ^

The error is introduced by the following commit:
https://reviews.llvm.org/rL348053

Basically, the clang FieldDecl method EvaluateAsInt signature got
changed and there is no compatible way in the llvm/clang
to also work in the old versions. So this patch just provided
a new implementation to get field value for llvm version 8 and later.
Signed-off-by: Yonghong Song <yhs@fb.com>