Support unresolved dynamic constants in CI.

ciConstant ciEnv::get_constant_by_index_impl(const constantPoolHandle& cpool,
                                             int pool_index, int cache_index,
                                             ciInstanceKlass* accessor) {
...
  } else if (tag.is_dynamic_constant()) {
    return ciConstant();
...

Right now, unresolved JVM_CONSTANT_Dynamic CP entries bail out compilations (both C1 and C2):

   201     410   91    b  1       compiler.runtime.TestConstantsInError$Test_CD_9/0x0000000800c11000::test (3 bytes)
   202 compilation bailout: could not resolve a constant
   203     410   91    b  1       compiler.runtime.TestConstantsInError$Test_CD_9/0x0000000800c11000::test (3 bytes)   COMPILE SKIPPED: could not resolve a constant (retry at different tier)


    296   64    b        Test_CD/0x0000000800c0a400::test (3 bytes)
    296   64    b        Test_CD/0x0000000800c0a400::test (3 bytes)   COMPILE SKIPPED: cannot parse method (retry at different tier)


src/hotspot/share/c1/c1_GraphBuilder.cpp:
void GraphBuilder::load_constant() {
  ciConstant con = stream()->get_constant();
  if (con.basic_type() == T_ILLEGAL) {
    // FIXME: an unresolved Dynamic constant can get here,
    // and that should not terminate the whole compilation.
    BAILOUT("could not resolve a constant");

src/hotspot/share/ci/ciTypeFlow.cpp:
// ciTypeFlow::StateVector::do_ldc
void ciTypeFlow::StateVector::do_ldc(ciBytecodeStream* str) {
  ciConstant con = str->get_constant();
  if (con.is_valid()) {
    ...
  } else {
    if (str->is_unresolved_klass_in_error()) {
      trap(str, NULL, Deoptimization::make_trap_request(Deoptimization::Reason_unhandled,
                                                        Deoptimization::Action_none));
    } else {
      // OutOfMemoryError in the CI while loading constant
      push_null();
      outer()->record_failure("ldc did not link");
    }