Revert "comment out the EvaluateAsInt compat code for now"

This reverts commit da9fb5d6d9ebf9363981c370ce937d8848989fcb, obsoleted by
<http://llvm.org/viewvc/llvm-project?view=revision&revision=347756> "Re-commit
r347417 'Re-Reinstate 347294 with a fix for the failures.'"
diff --git a/compilerplugins/clang/compat.hxx b/compilerplugins/clang/compat.hxx
index 7a51877..97410fc 100644
--- a/compilerplugins/clang/compat.hxx
+++ b/compilerplugins/clang/compat.hxx
@@ -157,8 +157,7 @@ inline bool CPlusPlus17(clang::LangOptions const & opts) {
}

inline bool EvaluateAsInt(clang::Expr const * expr, llvm::APSInt& intRes, const clang::ASTContext& ctx) {
// this keeps getting landed and then reverted in clang
#if 0 && CLANG_VERSION >= 80000
#if CLANG_VERSION >= 80000
    clang::Expr::EvalResult res;
    bool b = expr->EvaluateAsInt(res, ctx);
    if (b && res.Val.isInt())