Debug Info: enable verifying by default and disable testing cases that fail.

1> Use DebugInfoFinder to find debug info MDNodes.
2> Add disable-debug-info-verifier to disable verifying debug info.
3> Disable verifying for testing cases that fail (will update the testing cases
   later on).
4> MDNodes generated by clang can have empty filename for TAG_inheritance and
   TAG_friend, so DIType::Verify is modified accordingly.

Note that DebugInfoFinder does not list all debug info MDNode.
For example, clang can generate:
metadata !{i32 786468}, which will fail to verify.
This MDNode is used by debug info but not included in DebugInfoFinder.
This MDNode is generated as a temporary node in DIBuilder::createFunction
  Value *TElts[] = { GetTagConstant(VMContext, DW_TAG_base_type) };
  MDNode::getTemporary(VMContext, TElts)