This patch helps add the missing context `IsGNUStyle`. Before this patch, yaml2obj cannot parse the YAML description of 'debug_gnu_pubnames' and 'debug_gnu_pubtypes' correctly due to the missing context.
In other words, if we have
```
DWARF:
debug_gnu_pubtypes:
Length:
TotalLength: 0x1234
Version: 2
UnitOffset: 0x1234
UnitSize: 0x4321
Entries:
- DieOffset: 0x12345678
Name: abc
Descriptor: 0x00 ## Descriptor can never be mapped into Entry.Descriptor
```
yaml2obj will complain that "error: unknown key 'Descriptor'".
This patch helps resolve this problem.
Reviewed By: jhenderson
Differential Revision: https://reviews.llvm.org/D82435
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
Xing GUO