[ObjectYAML][DWARF] Collect diagnostic message when YAMLParser fails.

Before this patch, the diagnostic message is printed to `errs()` directly, which makes it difficult to use `FailedWithMessage()` in unit testing.
In this patch, we add a custom error handler for YAMLParser, which helps collect diagnostic messages and make it easy to use `FailedWithMessage()` to check error messages.

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D82630