Removes files for separate ReaderYAML.cpp and WriterYAML.cpp 
and adds a new file ReaderWriterYAML.cpp that uses YAML I/O.

Lots of tweaks to test suite for slightly different YAML encoding.


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@171592 91177308-0d34-0410-b5e6-96231b3b80d8