utils: Silence -Wpedantic warning

llvm/tools/clang/utils/TableGen/ClangAttrEmitter.cpp:1673:67: warning: default argument specified for lambda parameter [-Wpedantic]
                    const Record *Constraint = nullptr) {
                                               ^~~~~~~

Found by gcc 5.4.0.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@301783 91177308-0d34-0410-b5e6-96231b3b80d8